Learn in the textbookOpen Source Software (OSS) and Licenses
DefinitionCopyleft (コピーレフト) is the idea that when software is modified and passed on, the recipients still receive the same freedom to use, modify and redistribute it as the original gave.
ExplanationThe copyright holder uses their copyright to set a license, and the license requires that anyone passing the software on does so under the same terms. This stops a modified version from turning into a form without those freedoms partway along, so the freedoms carry on from one recipient to the next.
AnalogyIt is like an arrangement where someone given a seedling, if they share the plants they grow, must pass on the same promise they received: grow it freely and share it freely.
Memory hookThe name is wordplay that swaps the "right" in copyright for "left".