Wistfulness is a soft, quiet kind of sadness. You think about something you want but cannot have — very often something you HAD before, and lost. An old friend. The town you grew up in. The person you were at twenty. It is not crying-sad. It is the feeling of looking out of a train window and going quiet for a while.

Two things make it different from plain sadness. First, it is gentle. There is something almost sweet in it — you half enjoy the feeling. Second, it usually points backwards, to the past, or sideways, to a life you did not choose. If you are sad because you failed a test today, that is not wistfulness. If you are sad because you remember how happy you were before the test existed at all, that is.

The word is a noun for the feeling itself. It is a bit literary — you meet it in novels, in film reviews, in someone writing carefully about their own family. In normal speech people are far more likely to use the adjective: 'She looked wistful.' 'He said it wistfully.' If a student says 'I felt a great wistfulness', a native speaker will understand, but will also notice you have been reading books, which is not a bad thing.

People also use it about art and places, not just people: 'a wistfulness in the music', 'the wistfulness of an empty school in summer'. Here it means the thing gives you that feeling, or seems to have it itself.