{e-tex} is an extended version of TeX, from the NTS project.

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