standard conditions
When two cooks compare recipes, they need to agree on a standard oven temperature, or their notes will not match. Chemists face the same problem: a substance's measured properties depend on temperature and pressure, so to compare numbers fairly we agree on a fixed reference setting. Those agreed reference values are the standard conditions.
In practice, standard conditions specify a chosen temperature and pressure at which tabulated data are reported. A widely used set, standard temperature and pressure, fixes 0 °C and a defined pressure (in the modern convention, 100 kPa). Separately, when reporting thermodynamic data, chemists use a standard state — most commonly 1 bar of pressure and a stated temperature, usually 25 °C — to define quantities marked with a small circle, like the standard enthalpy of a reaction.
These conventions matter because they make published data comparable and reproducible across labs and textbooks. The honest pitfall is that several slightly different standards are in use, and older books often quote 1 atm rather than 1 bar; the difference is small but real, so it always pays to check which standard a number was measured against.
A textbook says one mole of an ideal gas fills about 22.4 litres — but only at standard temperature and pressure; warm it up or drop the pressure and the same mole swells to occupy more space.
Tabulated values hold only at the stated standard; change the conditions and they shift.
Do not confuse standard conditions (a reference temperature and pressure for reporting data) with the standard state (a reference condition for a substance used to define thermodynamic quantities); they overlap but answer different questions.