e-TeX – An extended version of TeX, from the NTS project

An extended version of TeX (capable of running as if it were unmodified TeX). E-TeX has been specified by the LaTeX team as the base engine for LaTeX2ε. Thus, LaTeX programmers may assume e-TeX functionality, along with additional extensions.

The pdftex engine and others directly incorporate the e-TeX extensions. The etex program in most distributions is an incarnation of pdftex running in DVI mode.

The development source for e-TeX is the TeX Live source repository, although further extensions have taken place in the pdftex and other engine sources, keeping e-TeX stable.