WordPress - Wikipedia
en.wikipedia.org
(That currently says 17 years ago, but it was today, the 27th, in 2003.)
~1/3 of the web is run with WP.
Also notable:
"WordPress is used by 41.4% of the top 10 million websites as of May 2021, WordPress is one of the most popular content management system solutions in use."
It says 'one of', but the source has WP as the largest except for "Other" which is a mix of different systems. So, it's the most popular singular CMS out there.