TeX is a markup language for typography, designed by Donald Knuth. It allows detailed control over layout and is particularly strong in its capabilities for mathematical formulas. It includes a full programming language.
The "X" in the name is properly pronounced like the Greek letter chi (or the German ch), though it's often pronounced as "tek." The name is properly typeset like "TEX", but "TeX" with a lowercase E is allowed as a surrogate.
TeX has a powerful macro facility, and several macro extensions of TeX, such as LaTeX, are widely used.
TeX is written in WEB.