Definition: Def. 11.20: Base of a Cone
(Definition 20 from Book 11 of Euclid's “Elements”)
And the base (of the cone is) the circle described by the (remaining) straight line (about the right angle which is) carried around (the axis).
Modern Formulation
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Definitions: 1
Proofs: 2
Propositions: 3
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References
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