Why is "key-rotation" called key-rotation?
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.