STP noun 6adjective 1forms 1 noun6 •(slang) A hallucinogen (2,5-dimethoxy-4-methylamphetamine), chemically related to amphetamine but with LSD-like effects.“Don,” she cries, breathless. “We got some STP today.” At this time STP is a pretty big deal, remember; nobody yet knew what it was and it was relatively, although just relatively, hard to come by. — 1968 •Initialism of standard temperature and pressure. •Initialism of sewage treatment plant. •Initialism of spanning tree protocol. •Initialism of shielded twisted pair. •Initialism of stand-to-pee. adjective1 •Initialism of subject to planning.This building would suit alternative uses STP. formsSTPs