Fair use / fair dealing
The legal doctrine allowing limited reuse of copyrighted material without permission, for purposes such as criticism, comment, teaching, and research.
US fair use weighs four factors: the purpose of the use, the nature of the work, how much you took and how central it was, and the effect on the market for the original. Fair dealing, in the UK, Canada, and Australia, works from a closed list of permitted purposes instead — narrower, and more predictable.
Neither is a blanket permission for academic work. Quoting a paragraph to analyse it sits comfortably inside both; reproducing a full-page figure because it illustrates your point well often does not.
See also
CopyrightThe legal right controlling copying and reuse of a creative work, which is separate from the academic duty to cite.Creative Commons licenceA family of standard licences by which a rightsholder grants reuse rights in advance, on stated conditions.Direct quotationReproducing a source's exact words inside quotation marks or as a block, with a citation that includes a locator.