Source: PAC, 1994, 66, 1077. (Glossary of terms used in physical organic chemistry (IUPAC Recommendations 1994)) on page 1118 [Terms] [Paper]
See also: Orange Book, 2nd ed., p. 227 [Terms] [Book] PAC, 1996, 68, 2223. (Glossary of terms used in photochemistry (IUPAC Recommendations 1996)) on page 2245 [Terms] [Paper]