e-TeX – 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). 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.