Wiktionary
n. (context psychology English) A rapid and extreme changing in mood, from excessively happy to desperately miserable.
Wikipedia
A mood swing is an extreme or rapid change in mood. Such mood swings can play a positive part in promoting problem solving and in producing flexible forward planning. However, when mood swings are so strong that they are disruptive, they may be the main part of a bipolar disorder.
Usage examples of "mood swing".
He spoke quietly but persuasively, and he sensed the mood swing in his favour.
Megan felt her mood swing crazily from black despair to a wild exhilaration.
Sooner or later, one of his hotties, during a bad mood swing, will maim or kill him.
Damug was smiling sweetly, his swift mood swing and calm tone indicating that his servant was out of danger, for the moment.
This mood swing was inappropriate, that orgasmic response pattern was a biochemical form of child abuse, a mother experiencing anger is damaging her child.
In a mood swing as abrupt as sun after a rain-squall, Orloff draped his big arm over Nessler's shoulders and walked the Manticoran back to the tent with him.
The mood swing had struck him so swiftly she was tempted to call it a revelation.