Word of the day: copyleft

I first came across the word 'copyleft' in a Computer Weekly article called The World of Wikinomics just over a year ago.

Here's the par in question:

Projects under that [WikiMedia Foundation] umbrella include Wikipedia (contributed to and edited by millions of people, and the fourth most visited website in the world) Wikibooks and Wikisource (out-of-copyright or "copyleft" texts) Wikimedia Commons (royalty-free multimedia files) Wikinews (which, like "microblogging" site Twitter, often breaks news globally), and Wikiversity (free-to-use courses and educational materials).

I'm not sure whether CW is saying here that Wikisource includes both out-of-copyright and copyleft texts, or whether it is saying that 'copyleft' means 'out of copyright'. I'm veering towards the latter.

So is copyleft a synonym for 'out of copyright'? Not according to Wikipedia:

Copyleft is a play on the word copyright to describe the practice of using copyright law to offer the right to distribute copies and modified versions of a work and requiring that the same rights be preserved in modified versions of the work.

Copyleft is a form of licensing and can be used to maintain copyright conditions for works such as computer software, documents, music and art.

Whatever its exact definition (if it has one), the word 'copyleft' originated in the 1970s and now has fairly widespread use: over seven million results in Google, although some of those are company names and so on. I'm surprised I only encountered it for the first time in 2009.

Oh, and just because I like it, here's a copyleft symbol: