Whim , caprice and vagary are all sudden and sometimes irrational notions, especially impulses to do something. A whim is a passing fancy or wish, often fantastic or odd.

• On the first day of spring, the whim struck him to wear a huge rose in his lapel; Queen Elizabeth I, although an expert in statecraft, tended in private to indulge in foolish whims .

Caprice , stemming from the Latin word for goat, is a sudden change of mood, opinion or purpose without apparent motivation. As implied by its etymology, caprice suggests an insistence upon having one’s own way: a kindergarten teacher trying to control the caprices of a wilful little girl; a patient whose various caprices against obeying the doctor’s orders delayed his recovery. Vagary originally meant a journey or a rambling about and still suggests strongly unpredictable, erratic or even irresponsible behaviour: the vagaries of an employee who is always late and frequently lazy. It is a stronger term than either whim or caprice . Whim , vagary and caprice may be used also in figurative senses: to sail at the whim of tide and wind; the caprices of fortune; the vagaries of spring weather.

A crotchet is a perverse idea or opinion, about a particular subject, held obstinately despite its obvious untruth or contradiction to common opinion.

• Mr. Midwick was well off, but his crotchet was that skimping on butter was a virtue.

Crotchets often concern minor points of doctrine or belief or merely trivial matter.

• There is no crotchet so ridiculous, nor any idea so silly, that it cannot find acceptance somewhere.

Whimsy , once considered a synonym of whim , is now applied almost exclusively to an odd, fanciful style of humour that delights and astonishes. Whimsy may appear in speech or actions, but is more likely to occur in literature: the whimsy of Lewis Carroll; the whimsy of The Wind in the Willows. Whimsy may range from charming fancy to tongue-in-cheek drollery.