Reductive materialism (identity theory) = the view that mental states are physical states of the brain (P 254a)

Just as:

Sound is "a train of compression waves traveling through the air" (P 254a)

Warmth is "high average molecular kinetic energy" (P 254b)

Lightning is "sudden large-scale discharge of electrons between clouds, or between the atmosphere and the ground" (P 254b)

So also:

A mental state is a brain state.