Monism 

Monism is the broader philosophical category that states there is only one kind of fundamental "stuff" or substance in the universe. Instead of an antithetical concept, materialism is just one possible answer to the question "What is that one fundamental substance?" 

Other forms of monism include:

• Idealist monism: The belief that everything is fundamentally mental or spiritual, with the physical world being a manifestation of consciousness.
• Neutral monism: The view that the single, fundamental substance of the universe is neither mental nor physical. Both mental and physical phenomena can be reduced to this neutral "stuff". 

Materialism 

Materialism, also known as physicalism, is the specific form of monism that holds that the only fundamental substance in the universe is matter or energy. According to this view, everything—including mental states like thoughts and consciousness—is caused by or is a result of physical processes.