Skip to Content


Pronunciation: or'der


  1. In biologic classification, the division just below the class (or subclass) and above the family.
  2. In a reaction, order is the sum of the exponents of all the concentration terms in that reaction's rate expression. For example, for the natural decomposition of nitrogen pentoxide, the rate expression is v = -d[N2O5]/dt = k1[N2O5]. Thus, this is a first-order reaction. A reaction involving two different compounds is often a second-order reaction (but not necessarily so). Pseudo–first-order reactions are multiorder reactions in which one of the reactants is in substoichiometric amounts.
  3. The sequence of residues in a heteropolymer.

[L. ordo, regular arrangement]

Further information

Always consult your healthcare provider to ensure the information displayed on this page applies to your personal circumstances.

© Copyright 2018 Wolters Kluwer. All Rights Reserved. Review date: Sep 19, 2016.