A whim is a sudden wish to do something or have something, when there is no real reason behind it. You are walking home, you pass a shop, and you suddenly want a cake — that wanting is a whim. The key idea is two things together: it comes fast, and it does not come from thinking. A plan has reasons. A whim has none, and often it is gone an hour later.

The most common way you will meet this word is in the phrase 'on a whim': 'She bought the ticket on a whim.' That means she did not plan it, did not think about it, just did it. You will also hear 'act on a whim' and, less often, 'a passing whim' (one that does not last).

There is a second use that feels different and is less friendly. When someone has power over you, their whims are their sudden wishes that you must follow. 'The workers were at the whim of the manager' means the manager could change his mind any time, and they had no choice. Here the word carries a small complaint: it says the wishes are not reasonable. You will also see 'every whim' — 'He gives that child her every whim' — meaning someone gets whatever they suddenly want, and the speaker thinks that is too much.

So the same word can be light and sweet (a small happy surprise in your own life) or a little bitter (someone else's power over you). The feeling comes from who owns the whim: yours is charming, your boss's is not. It is a normal word, not formal, and it is countable — one whim, two whims.