Stoichiometric Coefficient
The number written before a formula in a balanced equation, giving the relative amount of that substance consumed or produced.
Unit
Dimensionless
In depth
Coefficients apply to moles and to molecules, and for gases at constant temperature and pressure also to volumes. They must not be confused with subscripts, which describe composition within a formula. In thermochemical equations the coefficients also scale the quoted enthalpy change.
Examples in the real world
In N₂ + 3H₂ → 2NH₃ the coefficients 1, 3 and 2 mean one mole of nitrogen needs three of hydrogen.