Adjective

1. Allowing use, distribution, and modification on the condition that any redistributions or modifications also be licensed under the same condition and, in the case of computer software, are accompanied by source code

   The GNU General Public License is a copyleft licence.

Noun

1. A copyleft license.
2. The act of licensing under a copyleft license.

   I have placed my program under copyleft.

Verb

1. To license under a copyleft license.

   Linus Torvalds copylefted the Linux kernel.