standard temperature and pressure

Short definition from Wiktionary

Noun

1. 1
   One of various standard sets of conditions for experimental measurements used to aid reproducibility. Abbreviated as STP. Often elliptically called standard conditions.
2. 2
   The international, standard conditions used by the IUPAC to aid reproducibility; a temperature of 0 °C (273.15 K) and a pressure of 1 bar (100 kPa). Abbreviated as STP. Often elliptically called standard conditions.