Reductive materialism (identity theory) = the view that mental states are physical states of the brain (IP3 289a)

Just as:

Sound	is	"a train of compression waves traveling through the air" (IP3 289b)
Color	is	"a triplet of reflectance efficiencies" (IP3 289b)
Warmth	is	"high average molecular kinetic energy" (IP3 289b)
Lightning	is	"sudden large-scale discharge of electrons between clouds, or between the atmosphere and the ground" (IP3 289b)
So also:
A mental state	is	a brain state.