Longman Dictionary of Contemporary English
The Collaborative International Dictionary
Iso- \I"so-\, Is- \Is-\ [Gr. 'i`sos equal.] A prefix or combining form, indicating identity, or equality; the same numerical value; as in isopod, isomorphous, isochromatic. Specif.:
(Chem.) Applied to certain compounds having the same composition but different properties; as in isocyanic.
(Organic Chem.) Applied to compounds of certain isomeric series in whose structure one carbon atom, at least, is connected with three other carbon atoms; -- contrasted with neo- and normal; as in isoparaffine; isopentane.
Douglas Harper's Etymology Dictionary
word-forming element meaning "equal, similar, identical; isometric," from comb. form of Greek isos "equal to, the same as" (as in isometor "like one's mother"). Used properly only with words of Greek origin; the Latin equivalent is equi- (see equi-).
pre. 1 equal 2 (context chemistry English) forms terms relating to isomers 3 (context physics English) forms terms relating to isotopes