A WISP is a thin lock of hair, usually referring to a strand of hair that is particularly fine or delicate. It is often used to describe a person's hair when it is in a disheveled state, or when it is particularly light and airy. The term is also used to describe a small amount of something, such as smoke or fog. In the context of the crossword puzzle, the term 'WISP' is used to describe a thin lock of hair.