megacycle n.

before vowels meg-, word-forming element often meaning "large, great," but in physics a precise measurement to denote the unit taken a million times (megaton, megawatt, etc.), from Greek megas "great, large, vast, big, high, tall; mighty, important" (fem. megale), from PIE root *meg- "great." Mega began to be used alone as an adjective by 1982.

High-speed computer stores 2.5 megabits [headline in "Electronics" magazine, Oct. 1, 1957]

late 14c., cicle, "perpetual circulating period of time, on the completion of which certain phenomena return in the same order," especially and originally in reference to astronomical phenomena, from Old French cicle and directly from Late Latin cyclus, from Greek kyklos "circle, wheel, any circular body," also "circular motion, cycle of events," from PIE kw(e)-kwl-o-, a suffixed, reduplicated form of the root *kwel- (1) "to revolve, move round."

From 1660s as "any recurring round of operations or events" (as in life cycle). From 1821 as "single complete period in a cycle." Extended by 1842 to "any long period of years, an age." In literary use, "the aggregate of the legends or traditions around some real or mythical event or character" (1835).

By 1884 as "recurring series of oscillations or operations in an engine, etc." From 1870 as short for motorcycle; by 1881 as short for bicycle or tricycle.