neuroleptic

[French neuroleptique : neuro-, nerve (from Greek; see neuro-) + -leptique, affecting (from Greek lēptikos, seizing , from lēptos, seized , from lambanein, lēp-, to seize, take).]

A tranquilizing drug, especially one used in treating mental disorders.

Having a tranquilizing effect.

1. modifying psychotic behavior.
2. any drug that favorably modifies psychotic clinical signs; the main categories of neuroleptics include the phenothiazines, butyrophenones and thioxanthenes. Called also antipsychotic and major tranquilizer.
Drugs of this type stabilize mood and reduce anxiety, tension and hyperactivity. They are also effective in helping to control agitation and aggressiveness.

The noun has one meaning:

Meaning #1: tranquilizer used to treat psychotic conditions when a calming effect is desired
  Synonyms: major tranquilizer, major tranquillizer, major tranquilliser, antipsychotic drug, antipsychotic agent, antipsychotic, neuroleptic drug, neuroleptic agent