Some sample entries:

EXCITATION

1 In physiology, a process whereby some stimulus energy pattern sets up a change or pattern of changes in a receptor. The energy here may be either physical or other neutral activity; see STIMULATION. 2 In the study of learning, a general high level of activity in the whole nervous system; a reasonable synonym here is drive state. 3 In social psychology, an increase in psychological tension; this meaning is intuitively close to conventional usage.

TROXLER'S EFFECT

A visual phenomenon. When an observer maintains fixation on a point directly in front while attempting to view a stationary line off to one side, this peripheral stimulus disappears. It is especially striking at low illumination levels but occurs at high levels as well, when a sort of visual fog seems to creep in from the periphery, obscuring objects. A slight movement in the periphery causes the peripheral stimuli to reappear.

SEROTONIN

A neurotransmitter found in neural pathways of peripheral ganglia and in the central nervous system. Also known as 5-hydroxytryptamine or 5-HT, it is an inhibitory transmitter the actions of which have been implicated in various processes, including sleep, pain, and the psychobiology of various affective disorders, specifically depression and bipolar disorder. At the time or writing, at least nine different types of serotonin receptors have been identified. Interestingly, while serotonin is involved in mediating many important behaviours, only some 1-2% of the body's serotonin is found in the nervous system: most is in the mucous membranes of the gastrointestinal system and blood platelets.