Copyleft License
Copyleft (from the English copyleft) is a license that: allows original works to be used in creating new (derivative) works without obtaining permission from the copyright holder; and requires that these same conditions be preserved in the license of the derivative work.
The term “copyleft” © comes from the term “copyright” ©.
By using copyleft licenses, authors and rights holders grant the right to distribute copies of the original work and its modified versions. Authors of derivative works are required to distribute them while preserving the same rights as the original work. As a result, the use of a copyleft license makes it possible to create a free work.
Such a work may be, for example, a computer program. In that case, all modified and extended versions of the program must also remain free.