A solution of accurately known concentration, prepared using standard substances in one of several ways. A primary standard is a substance of known high purity which may be dissolved in a known volume of solvent to give a primary standard solution. If stoichiometry is used to establish the strength of a titrant, it is called a secondary standard solution. The term secondary standard can also be applied to a substance whose active agent contents have been found by comparison against a primary standard. Concentrations of standard solutions may be expressed in kmol m-3, mol dm-3, or in terms more closely related to those used in specific titrations (as titres).
Orange Book, 2nd ed., p. 48 (https://media.iupac.org/publications/analytical_compendium/)