Each materialist theory is distinguished from other materialist theories by the way it answers this question. Every mental term refers to a disposition to behave in a particular observable manner under certain observable conditions. The truth or falsity of any statement using mental terms can be empirically determined. The inner aspect of our experience is left out. A truly comprehensive behaviorist definition of a mental term must make use of other mental terms, creating a vicious circle.
Every mental term refers to a type of physical state of the human brain and nervous system.
Mental states are actually brain states. Faces hopeless task of matching vague psychological terminology to precise neuroscientific descriptions. Not biological, so psychology as a separate science is preserved. Leaves the qualitative nature of experience out of the description of a mental state. Like Identity Theory, tied to our current psychological conceptual framework.