1. Chemical prefix denoting 1) elimination of one methylene group from a chain, the highest permissible locant being used; 2) contraction of a (steroid) ring by one CH2 unit, the locant being the capital letter identifying the ring. Elimination of two methylene groups is denoted by the prefix dinor-; three groups, by trinor-, norleucine. 2. Chemical prefix denoting “normal,” unbranched chain of carbon atoms in aliphatic compounds, as opposed to leucine, a branched chain with the same number of carbon atoms.