What is "patentleft"

Patentleft (also patent left, copyleft-style patent license) is the practice of licensing patents (especially biological patents) for royalty-free use, on the condition that adopters license related improvements they develop under the same terms. Copyleft-style licensors seek "continuous growth of a universally accessible technology commons" from which they, and others, will benefit.

Patentleft is analogous to copyleft, a license which allows distribution of a copyrighted work and derived works, but only under the same terms.