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]

Disclaimer: This site is designed to offer information for general educational purposes only. The health information furnished on this site and the interactive responses are not intended to be professional advice and are not intended to replace personal consultation with a qualified physician, pharmacist, or other healthcare professional. You must always seek the advice of a professional for questions related to a disease, disease symptoms, and appropriate therapeutic treatments.
© Copyright 2018 Wolters Kluwer. All Rights Reserved. Review Date: Sep 19, 2016.