An extended version of TeX (which is capable of running as if it were TeX unmodified). E-TeX has been specified by the LaTeX team as the engine for the development of LaTeX 2e, in the immediate future; as a result, LaTeX programmers may (in all current TeX distributions) assume e-TeX functionality.

Development versions of e-TeX are to be found in the TeX live source repository.

The author is Peter Breitenlohner.