A
depressant, or
central depressant, is a
drug that lowers
neurotransmission levels, which is to depress or reduce
arousal or
stimulation, in various areas of the brain. Depressants are also occasionally referred to as "downers" as they lower the level of arousal when taken.
Stimulants or "uppers" increase mental and/or physical function, hence the opposite
drug class of depressants is stimulants, not
antidepressants.