Certainly. Schelling’s vocabulary shifts considerably across his career, so some terms mean different things in the Naturphilosophie, the Identity Philosophy, the 1809 Freedom Essay, and the later philosophy of mythology/revelation. The following is therefore a working dictionary, with the German terms where useful.

A short dictionary of Schellingian terms

Term	Brief definition
Abgrund — abyss, bottomlessness	Literally “groundless depth.” That which lacks a determinate ground or foundation. Schelling sometimes uses abyssal language for what precedes or exceeds determinate being and rational articulation. Closely related to, but not always simply synonymous with, the Ungrund.
Absolute (das Absolute)	Ultimate reality considered prior to the divisions between subject/object, ideal/real, mind/nature. In the Identity Philosophy, the Absolute is the indifference or identity of these opposites rather than one side dominating the other.
Being (Sein)	What is. In later Schelling, particularly important because Schelling increasingly distinguishes the fact that something exists from the rational determination of what it is. This distinction helps motivate his critique of purely “negative” philosophy.
Contraction (Kontraktion / Zusammenziehung)	A primordial drawing-together or self-contraction through which something acquires particularity, solidity and selfhood. Particularly important in the Freedom Essay and Ages of the World. Schelling associates contraction with the dark, grounding principle of existence.
Dark ground (dunkler Grund)	The obscure, pre-rational ground underlying conscious, ordered existence. In God, nature and human beings, rational manifestation arises from something that is not itself transparent reason. Hence Schelling's famous idea that there is in God a ground of God's existence which is nevertheless not God insofar as God exists.
Existence (Existenz)	Not merely “being there,” but manifested or actual being. In the Freedom Essay, Schelling distinguishes a being's ground of existence from the being insofar as it actually exists.
Expansion	The outward, expressive tendency opposed to contraction. If contraction produces inwardness, particularity and selfhood, expansion produces manifestation, communication, light and universality. Their dynamic opposition becomes central in the Ages of the World.
Freedom (Freiheit)	Not merely freedom from external constraint. Genuine freedom involves the real possibility of good and evil. For Schelling, a person is free because the principles constituting personality can be ordered or disordered rather than operating mechanically.
Ground (Grund)	One of Schelling's most important concepts: that from which something arises or through which it is what it is. Crucially, ground is not necessarily identical with cause. Something can be the ground of another thing without temporally preceding it.
Ground of existence (Grund der Existenz)	The underlying basis from which an existing being emerges. In the Freedom Essay, Schelling distinguishes God's existence from the ground in God of God's existence. This ground is “nature in God”: obscure desire or longing prior to fully articulated divine personality.
Groundlessness / Unground (Ungrund)	Perhaps the term you mean by Urgrund. The Ungrund is that which precedes the distinction between opposed principles and therefore cannot itself possess a determinate ground. In Schelling's mature thought, it names a primordial indifference prior to the opposition of ground/existence, darkness/light, etc. The term has an important precedent in Jakob Böhme.
Urgrund — primal ground	Literally the “original” or “primordial ground.” The term is sometimes used in discussions of Schelling for the ultimate or primordial ground, but Ungrund is the more philosophically distinctive Schellingian term, especially in connection with the Freedom Essay. Urgrund suggests a first/deepest ground; Ungrund means the absence or transcendence of ground.
Indifference (Indifferenz)	Not apathy. It means a primordial state in which opposites are not yet differentiated. Thus the Absolute can be described as the indifference of subject and object, ideal and real. In the Freedom Essay, the Unground is an indifference prior to the differentiation of the two fundamental principles.
Identity (Identität)	The deeper unity underlying apparent opposition. Schelling's “philosophy of identity” argues that subject and object, nature and spirit, ideal and real must ultimately belong to one reality. Identity therefore does not simply mean that the two sides are indistinguishable.
Potency (Potenz)	A fundamental power, level or structural principle through which the Absolute manifests itself. Schelling frequently describes reality as an ordered series of potencies. The precise scheme changes considerably over his career.
Longing (Sehnsucht)	A primordial striving or desire. In the Freedom Essay, Schelling speaks of the longing felt by the eternal One “to give birth to itself.” Longing belongs to the obscure ground preceding fully articulated understanding and personality.
Nature (Natur)	Not merely the collection of physical objects. Nature is productive, dynamic activity: an unconscious productivity that progressively gives rise to organized life and ultimately consciousness. Hence Schelling's famous conception of nature as visible spirit and spirit as invisible nature.
Naturphilosophie	Schelling's “philosophy of nature.” An attempt to understand nature as an autonomous, productive process rather than merely as an object constructed or represented by the human subject.
Natura naturans / natura naturata	Traditional terms Schelling inherits largely through Spinoza: nature naturing versus nature natured. The former is nature as productive activity; the latter is nature considered as the products generated by that activity. Schelling strongly privileges understanding nature through its productivity.
Productivity (Produktivität)	Nature considered as active production rather than as a collection of finished products. A fundamental concept of the Naturphilosophie: every apparently stable natural object is the temporary stabilization of underlying dynamic forces.
Polarity (Polarität)	The principle according to which natural and spiritual processes develop through opposed but mutually dependent forces—attraction/repulsion, contraction/expansion, real/ideal, etc. Opposition is productive rather than simply contradictory.
Real / Ideal (das Reale / das Ideale)	Two fundamental dimensions of reality. The real tends toward objectivity, nature and particularity; the ideal toward subjectivity, spirit and intelligibility. Schelling seeks their ultimate unity rather than reducing one to the other.
Spirit (Geist)	Nature becoming conscious of itself. Spirit is therefore not simply an alien substance inserted into nature: the emergence of consciousness reveals something that was implicitly present in nature's own productivity.
Intellectual intuition (intellektuelle Anschauung)	A direct apprehension of the identity underlying subject and object. Early Schelling inherits the term from post-Kantian debates but gives it an especially important role in apprehending the Absolute.
The One / eternal One (das Eine)	The primordial unity underlying differentiation. Especially in Schelling's middle period, the One does not remain sterile unity but differentiates itself into opposed principles through which personality and revelation become possible.
Understanding (Verstand)	In the Freedom Essay, not merely ordinary discursive intellect. The divine understanding is associated with articulation, order, universality and “light,” in contrast to the obscure longing of the ground.
Will (Wille)	Increasingly fundamental in Schelling's middle and later philosophy. Reality at its deepest level is not adequately understood as static substance or representation; it involves will, activity and becoming. Schelling famously writes that, in the final and highest instance, there is no being other than willing.
Selfhood (Selbstheit)	The principle through which a being becomes a distinct individual centre. Selfhood requires contraction and separation, and is therefore necessary for personality and freedom—but it also creates the possibility of evil when the individual principle seeks supremacy over the universal principle.
Evil (das Böse)	Not simply absence, ignorance or privation. Schelling insists that evil must be positively real if freedom is real. Evil arises when selfhood or the dark principle of particularity attempts to subordinate the universal principle to itself.
Personality (Persönlichkeit)	Genuine individuality produced through the living unity of opposed principles. Schelling's God is not merely an abstract Absolute but becomes intelligible as a living, personal God through the dynamic relation of ground and existence.
Revelation (Offenbarung)	The manifestation of what was initially hidden or implicit. This structure appears throughout Schelling: nature reveals spirit; consciousness reveals unconscious productivity; God reveals Godself; and, in the late philosophy, historical revelation acquires explicit philosophical significance.
Theogony	Literally the “coming-to-be of the gods.” In the later philosophy of mythology, mythological consciousness is understood as a real historical process through which divine potencies successively manifest themselves, rather than simply as primitive people inventing fictional stories.
Negative philosophy (negative Philosophie)	Philosophy proceeding through reason and conceptual necessity—roughly, philosophy determining what things must be in thought. Schelling's late critique is that reason can determine essences and possibilities but cannot, by itself, derive the sheer fact that reality actually exists.
Positive philosophy (positive Philosophie)	Schelling's late attempt to begin from actual existence rather than derive existence from concepts. It concerns contingent actuality, history, mythology and revelation—what is actually the case, rather than merely what is rationally possible or necessary.
The unprethinkable (das Unvordenkliche)	A major late Schellingian notion: that which is prior to thought and cannot be derived from thought. Existence ultimately confronts reason as something already given—something reason cannot completely produce from its own concepts.
Ecstasy of reason (Ekstase der Vernunft)	The point at which reason encounters what it cannot generate from itself. Reason must, as it were, stand “outside itself” before sheer existence. This becomes important to Schelling's transition from negative to positive philosophy.

The crucial cluster: Grund, Urgrund, Ungrund and Abgrund

These are especially easy to collapse into one another, but it is useful to keep their literal structure in view:

Grund = ground — a basis, foundation, or that from which something proceeds.

Ur-grund = primal/original ground — the most primordial ground.

Un-grund = un-ground / groundlessness — not simply another, deeper ground, but what is without determinate ground and precedes the distinction of principles.

Ab-grund = abyss — literally something like the ground that falls away beneath us; hence an unfathomable or bottomless depth.

The Ungrund is particularly important in the Freedom Essay. Schelling has distinguished two principles: ground and existence, darkness and light, selfhood and love. But if these really are principles within one absolute being, there must be something logically prior to their opposition. Schelling calls this the Ungrund.

The subtle point is that the Ungrund cannot simply be a third thing standing above the two principles. If it were, we would merely have introduced another determinate principle requiring explanation. It is instead indifference (Indifferenz): not the synthesis of already-existing opposites, but their condition before they have become opposites.

That gives a characteristic Schellingian movement:

Ungrund → differentiation → Ground / Existence → their living unity

or, in another vocabulary:

indifference → polarity → darkness / light → personality

This is one of the reasons Schelling becomes so important for later existential and psychoanalytic thought. The rational, conscious, articulated self is not self-grounding. Beneath reason lies something pre-rational—will, longing, nature, ground—which reason requires but can never make completely transparent to itself.

And this also marks an important difference from a simplistic picture of German Idealism as claiming that “everything is ultimately Reason.” In the Schelling of 1809 onward, reason discovers that being has a depth that cannot simply be dissolved into rational self-consciousness.

If you're reading the 1809 Philosophical Investigations into the Essence of Human Freedom specifically, the dozen terms to have immediately at hand are Grund, Existenz, Ungrund, Indifferenz, Sehnsucht, Wille, Verstand, Selbstheit, Freiheit, das Böse, Liebe, and Persönlichkeit. Almost the entire metaphysical argument can be reconstructed from the relations among those concepts.