mid-14c., monarchie, "a kingdom, territory ruled by a monarch;" late 14c., "rule by one person with supreme power;" from Old French monarchie "sovereignty, absolute power" (13c.), from Late Latin monarchia, from Greek monarkhia "absolute rule," literally "ruling of one," from monos "alone" (from PIE root *men- (4) "small, isolated") + arkhein "to rule" (see archon). Meaning "form of government in which supreme power is in the hands of a monarch" is from early 15c.
Tyranny is the abuse of absolute power, legal or usurped, and implies oppression. Despotism, in its earlier and still frequent meaning, does not necessarily imply either regard or disregard for the welfare of the subject; but there is also a tendency to give it essentially the same meaning as tyranny, using absolutism or autocracy where an unfavorable meaning is not intended. [Century Dictionary, 1897]