Temperamental describes someone or something that's unpredictable and moody, changing behavior suddenly for no clear reason. A temperamental person might be cheerful one moment and irritable the next, making it hard to know what to expect. Your friend might be temperamental if she's sometimes warm and friendly but other times snaps at you over small things, and you never quite know which version you'll get.

The word also applies to machines and equipment that work inconsistently. A temperamental printer might work perfectly one day but jam and refuse to print the next, even though you're doing everything the same way. Musicians talk about temperamental instruments that go out of tune easily or require constant adjustment.

When you call something temperamental, you're pointing out its unreliability. It's not steadily bad (that would be broken) or consistently good (that would be dependable). Instead, it keeps you guessing. A temperamental car starts some mornings but not others. A temperamental cat purrs when you pet her sometimes but hisses at you other times.

Someone with a calm temperament stays even and steady, while someone with a volatile temperament shifts moods frequently. When we say a person is temperamental, we mean their moods swing unpredictably, which can make them difficult to work with or be around.