Definition: Def. 11.23: Bases of a Cylinder
(Definition 23 from Book 11 of Euclid's “Elements”)
And the bases (of the cylinder are) the circles described by the two opposite sides (of the rectangle constructing the cylinder, which are) carried around.
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Adapted from (subject to copyright, with kind permission)
 Fitzpatrick, Richard: Euclid's "Elements of Geometry"
Adapted from CC BYSA 3.0 Sources:
 Prime.mover and others: "Pr∞fWiki", https://proofwiki.org/wiki/Main_Page, 2016