The International System of Units (SI) defines the amount of substance to be proportional to the number of elementary entities present. Both of these ensembles are very difficult to define because their associated elementary entities are extremely diverse and variable.
Air is the stuff humans breathe as they walk across a field.
Other organizations have established a variety of alternative definitions for their standard reference conditions.
In chemistry, IUPAC established standard temperature and pressure (informally abbreviated as STP) as a temperature of 273.15 K (0 °C, 32 °F) and an absolute pressure of 100 k Pa (14.504 psi, 0.987 atm, 1 bar).
mole is defined as the amount of substance that contains an equal number of elementary entities as there are atoms in 12g of the isotope carbon-12.