Hair wave is a hairstyle that involves the use of chemicals to permanently alter the structure of the hair. The term 'hair wave' is often used interchangeably with 'perm, ' which is a 4-letter word that is one of the solutions for this crossword. A perm involves the use of chemicals to break down the bonds in the hair, allowing it to be reshaped into a desired style. This style can last for several months and is often used to add volume and texture to the hair.