Genius is a general purpose calculator program similar in some aspects
to BC, Matlab, Maple or Mathematica. It is useful both as a simple
calculator and as a research or educational tool. The syntax is very
intuitive and is designed to mimic how mathematics is usually written.

GEL is the name of its extension language, it stands for Genius
Extension Language, clever isn't it? In fact, many of the standard
genius functions are written in GEL itself.