A TeX-integrated graphics language written by John D. Hobby from 1989 to 1994. It is useful for creating figures in technical documents, including but not limited to geometric figures, graphs, curved arrowheads, and Feynman diagrams. The output is typically more attractive than what you might produce with other graphics languages (Mathematica, for example), but the language is difficult to learn.
From Hobby's MetaPost page ( http://cm.bell-labs.com/who/hobby/MetaPost.html ), which was last updated in 1996:
MetaPost is a powerful graphics language based on Knuth's METAFONT, but with PostScript output and facilities for including typeset text. Details on the language itself are available in various publications. The language and its implementation are now in the public domain.