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by rcoveson 1299 days ago
Copyleft[0] accounts for a significant but minority fraction of free software. The GPL is a Copyleft licesense, but the MIT and Apache licenses (which together might account for a majority of free software) are not. Copyleft refers to the restriction that derivative works also be subject to the same Copyleft terms; it's the "infectious" nature of the license.

Having any license requirements at all, e.g. attribution, inclusion of license alongside distributed copies, etc. does not make the license Copyleft.

0. https://en.wikipedia.org/wiki/Copyleft