The Free On-line Dictionary of Computing (30 December 2018):
A GNU documentation system that uses a single source file to
produce both on-line information and printed output. You can
read the on-line information, known as an "Info file", with
an Info documentation-reading program. By convention, Texinfo
source file names end with a ".texi" or ".texinfo" extension.
You can write and format Texinfo files into Info files within
GNU Emacs, and read them using the Emacs Info reader. If
you do not have Emacs, you can format Texinfo files into Info
files using "makeinfo" and read them using "info". TeX is
used to typeset Texinfo files for printing.
Texinfo is available from your nearest GNU archive site.