Here is a more extensive definition:

Joomla is a free, award-winning content management system written in PHP which allows users to easily publish their content on the world wide web and intranets. Joomla is created as an open-source project where individuals and teams contribute their skills to its development as well as its supporting systems.
While Joomla is packed with features, its greatest quality is that it is extremely extensible. Because of its extensible structure, there aren't many things you cannot do with Joomla. A short list of some important features of Joomla are the following:

Page Caching
Web Indexing
RSS Feeds
Printable Page Versions
Newsflashes
Blogs
Forums
Polls
Calendars
Website Searching
Language Internationalization