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.

Source: Webster's Revised Unabridged Dictionary (1913)