Is-

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.: (a) (Chem.) Applied to certain compounds having the same composition but different properties; as in isocyanic. (b) (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. [1913 Webster]