STP (or NTP) is abbreviation for standard (normal) temperature (273.15 K or 0 °C) and pressure (105 Pa). These conditions are used when comparing the properties of gases.
Citing this page:
Generalic, Eni. "Stp." Croatian-English Chemistry Dictionary & Glossary. 29 June 2022. KTF-Split. {Date of access}. <https://glossary.periodni.com>.