n. copyleft /kop'ee-left/
A pun on copyright, copyleft is a legal agreement that has as its goal what could be considered the opposite of a copyright in a sense. While a copyleft is legally appicable to anything that's copyrightable, it is usually used in reference to free software.
The idea of a copyleft was intended to fix some of the problems (real or perceived) with releasing content to the public domain. The main problem with this is that by making a work public domain, you are allowing others to make changes, one or many, and then sell your work as their property. This clearly might cause some anger to the original author. A copyleft can prevent this (though it doesn't need to - see the following section).
There is often a great deal of confusion regarding the word "free" and the concept of copyleft. In this type of context, the word free can have two separate meanings, often defined as free as in beer and free as in speech. Since the most common application of copyleft licensing is in software, that is the example that will be used.
Free as in beer is just that. You never have to pay anything for it. Software that is distributed in this nature is not automatically under a copyleft, it's just free.
Software that is free as in speech means that you are free to modify the work to suit your needs. Most importantly, you are free to recompile it and distribute, and depending on which licence was used, you may even mark the version with your changes as proprietary (although in general this sort of license requires attribution assurance). This sort of license does generally constitute a copyleft agreement.
The Open Source Example
The notion of a copyleft is at heart deeply connected with the notion of open source. Perhaps the most famous example of a copyleft license, the GPL(General Public License) is targeted towards the notion of software, and is used in all of the licenses of the Free Software Foundation.
By excluding some of his or her own rights, the author of copylefted content says in effect "I will give this work to the global community". But here's where things get complicated. There are actually very many different versions of licenses that fall under the copyleft umbrella, spanning from the Academic Free License to the Zope Public License (If you're looking for a listing of licenses as such, check out the open source initiative at www.opensource.org/licenses/ ).
From the perspective of an end user, this is great. Free software. However, for the developer, the distinction between the two frees can be a little problematic - if they are intenting to distribute commercial software, they are barred from using free(speech) software even as a library, because to distribute any commercial work would be illegal. The GPL is often described as a "viral" license due to this effect. Anything that it touches becomes GPLed. A version of the GPL which does not have this viral effect is known as the LGPL(Lesser General Public License).
Another copyleft type movement that has recently become popular for works other than software is Creative Commons, which is covered in depth on its own node.