A hair unit is a type of hairpiece or wig that is used to add volume, length, and/or color to a person's natural hair. The term 'hair unit' can refer to three different types of hairpieces: TRESS (5 letters), STRAND (6 letters), and HANK (4 letters). A TRESS is a single piece of hair that is attached to the scalp with clips or glue. A STRAND is a group of several tresses that are connected together. A HANK is a large bundle of hair that is usually attached to the scalp with a net or cap.