H. Jerome Keisler

American logician who has made major contributions to model theory — particularly ultraproducts and nonstandard analysis — and co-authored the standard model theory textbook.

Ultraproducts: a construction that takes a product of structures modulo an ultrafilter — used to give an algebraic proof of the compactness theorem. Nonstandard analysis: developed Keisler's analysis textbook using Robinson's infinitesimals. Model Theory (with Chang): the standard reference text.