from
The Collaborative International Dictionary of English v.0.48
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]