From The Free On-line Dictionary of Computing (19 January 2023):
Compiled HTML
chm
<filename extension> A Microsoft file format for
distributing a collection of HTML files, along with their
associated images, sounds, etc., as a single compressed
archive file.
Microsoft use this format for Windows HTML Help files.
Most chms include a project (.hhp) file listing the included
files and basic settings, a contents (.hhc) file, an index
(.hhk) file, html files, and, optionally, image files.
Users view chms with hh.exe, the HTML Help viewer installed
with Internet Explorer.
Filename extension: .chm.
(http://msdn.microsoft.com/library/en-us/htmlhelp/html/vsconHH1Start.asp).
(2003-05-17)