Euclid's Elements, Book XI, Definitions 21 through 23 (original) (raw)

Definition 21

When a rectangular parallelogram with one side of those about the right angle remains fixed is carried round and restored again to the same position from which it began to be moved, the figure so comprehended is a cylinder.

Definition 22

The axis of the cylinder is the straight line which remains fixed and about which the parallelogram is turned.

Definition 23

And the bases are the circles described by the two sides opposite to one another which are carried round.