plain tex

from The Free On-line Dictionary of Computing (8 July 2008)

plain TeX

   <publication> {Donald Knuth}'s original set of user-level
   {macros} for interaction with his {TeX} formatter.  Dedicated
   TeX fans still prefer these over the more {user-friendly}
   {LaTeX} macros used by the majority of the TeX community.

   (1997-11-20)