Why is "key-rotation" called key-rotation? The word "rotate" in respect to "key-rotation" has connotations of looping/repeating variable values like a rotor, but as far as I can tell there isn't a single definition available for "key-rotation" that involves re-using a key that was previously replaced. 

So why don't we call it key-replacement or something else like that? i.e., why did the cryptography communities decide the more ambiguous word "rotation" was the more appropriate term to describe the act of changing keys?

It just seems a strange phrase to use, especially if you're not a native English speaker.

The only thing I could think of was some sort of an anachronism in reference to oldschool cryptographic key-generators, like the Enigma Machine, or your typical multi-rotor-combo-locks popular for use with bikes &amp; storage lockers.
Purely speculation, but given most cipher usage historically was military, diplomatic and espionage, I wonder if it might be related to the longtime common practice of the military (certainly US, and I believe many others), and to an extent the diplomatic corps at least US, of changing the assignments of individuals to jobs, posts, or duties, and for the military of whole units to missions, on a regular schedule automatically (i.e. not requested by the people involved). This practice is usually called rotation, and a particular (limited-time) assignment is metonymically called a rotation -- e.g. a soldier might speak of 'doing a 3-month rotation in \$job1 and then being moved to \$job2'. Random example To a military comms specialist -- and their commanding officer -- the periodic, scheduled replacement of one key by another would certainly seem similar.