Akismet is a spam filtering service that filters spam from comments, trackbacks, and contact form messages. The filter works by combining information about spam captured on all participating sites, and then using those spam rules to block future spam. Akismet is offered by Automattic, the company behind WordPress.com.
In late 2005, Mullenweg launched the Akismet plug-in for WordPress. Akismet (which stands for Automatic kismet) is a way of dealing with spam in aggregate, a crowd-sourced spam-fighting tool. Each time someone posts a comment to a participating website, Akismet checks it against all the comments in the database. If it is spam, it is deleted. If spam gets through and a user marks it as spam, the comment is added to the database so the pool of spam comments widens, making Akismet increasingly effective over time.
Akismet was originally developed to integrate with a plug-in for WordPress. The Akismet plug-in has been included by default in all WordPress builds since version 2.0 and activated in all WordPress.com-hosted blogs.
To use Akismet, it is necessary to obtain an API key. This key is necessary for a site to communicate with the Akismet servers.
Released under the terms of the GNU General Public License, the Akismet plug-in is free software, although the code to the Akismet system itself (and the nature of the algorithm used) has not been released. The Akismet service is free for personal use and, as of March 2016, commercial plans start at $5/month.