A professional approach:
Leslie Lamport, developer of LaTeX, called it:
LaTeX – A document preparation system
as is visible on the LaTeX project website. Moreover, your first slide could include the fundamental use of LaTeX (also from the above reference):
LaTeX is a high-quality typesetting system; it includes features designed for the production of technical and scientific documentation. LaTeX is the de facto standard for the communication and publication of scientific documents. LaTeX is available as free software.
A different approach:
The TeX Book mentions this about TeX:
It is important to understand the idea of token lists, if you want
to gain a thorough understanding of TeX, and it is convenient to learn
the concept by thinking of TeX as if it were a living organism. The
individual lines of input in your files are seen only by TeX's "eyes"
and "mouth"; but after that text has been gobbled up, it is sent to
TeX's "stomach" in the form of a token list, and the digestive processes
that do the actual typesetting are based entirely on tokens. As far as the
stomach is concerned, the input flows in as a stream of tokens, somewhat
as if your TeX manuscript had been typed all on one extremely long line.
LaTeX - Your document cookie monster