A concise definition of Drupal is difficult to come by, as many people use Drupal for many different things. The following definitions provide an incomplete cross-section of how different people use Drupal. Our working definition is the final one in the list.
Drupal is a database-driven web application written in PHP.
Drupal is an open-source Content Management System (CMS) freely available under the GPL.
Drupal is a community-building platform.
Drupal is a web development framework. You can use Drupal as a platform to build a broad range of web applications.
Note
The above definitions, however, can also benefit from further explanation. For those interested in additional reading and background, the following links provide a more detailed overview, and some background information:
PHP: http://php.net/
Web content management system, defined: http://en.wikipedia.org/wiki/Web_content_management_system
The GPL, or the Gnu Public License: http://www.gnu.org; Drupal is covered under version 2 of the GPL: http://www.gnu.org/licenses/old-licenses/gpl-2.0.html
Web development framework, defined: http://en.wikipedia.org/wiki/Web_application_framework
Background information on Open Source: http://www.opensource.org
Overview section, from the Drupal handbook: http://drupal.org/node/21951
Our definition: Drupal is a tool that helps people build interactive websites. It is free to download, install, customize, and use.