Copyleft

Copyleft indicates an opposite attitude to the significant restrictions which copyright imposes on the reader (despite the allowances of "fair use").

A copyleft license uses copyright law to assert that the material can only be further distributed under a similar license (i.e., one allowing redistribution).

An author can choose from a range of copyleft licenses (or make up his or her own) which are far more open and preferable to default copyright licenses.