Period - the period of the substitution cipher is the number of times that must be applied before the original message is returned.
What are the periods of ROT-n for each possible value for n?
Huh this is new. I'm not a cryptology person myself but I'm sure we could find an expert on this ever-expansive site.
(Doesn't cryptology involve riddles and encoding messages? The most I know about those are the Caesar and Kryptos)