Source : Webster's Revised Unabridged Dictionary (1913)

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.

Is- \Is-\
   See {Iso-}.