Relating to the chemical content of the brain and the surrounding cerebrospinal fluid. These chemicals include hormones that influence everything from sexual development to water content of the body, as well as neurotransmitters that allow neurons to communicate, giving rise to all thought, perception, and emotion, as well as sensation and movement.
This term may also be used as a noun.

In psychology, people sometimes form a dichotomy between neurochemical solutions and psychological solutions. While drugs may be more directly neurochemical than talking, my stance is that neurochemistry and psychology are intimately intertwined, perhaps identical. Anything that you experience will be reflected in your brain, and anything big that happens to your brain will affect you.