A small wig used to cover the top or crown of the head is known as a:

A small wig designed to cover just the top or crown of the head is referred to as a toupee. This term specifically describes a type of hairpiece that is generally intended to conceal hair loss or create the appearance of fuller hair in that particular area. Toupees are typically more limited in coverage compared to full wigs, which encase the entire head.

In contrast, a cap generally refers to the foundation or base used for various wig constructions, while a switch is a hairpiece that can be attached for extra length or volume but may not specifically cover the crown. Wefts are strips of hair that are sewn together and used in various hair extension techniques but do not describe a small wig. Understanding these definitions highlights why toupee is the correct response in this context.