see iso-.
2. is- ·- applied to certain compounds having the same composition but different properties; as in isocyanic.
3. is- ·- a prefix or combining form, indicating identity, or equality; the same numerical value; as in isopod, isomorphous, isochromatic.
4. is- ·- 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.