LyX is an open source document processor based on the LaTeX typesetting system. LyX is an advanced open source document processor running on Linux/Unix, Windows, and Mac OS X. LyX is a document processor that encourages an approach to writing based on the structure of your documents (WYSIWYM) and not simply their appearance. LyX is an open source document processor based on the LaTeX typesetting system. I always recommend that you learn the basics of LaTeX before you learn LyX. What is a LaTeX editor? Before addressing what a LaTeX editor is, let us see what TeX and LaTeX are.