What is the name given to the curly or wavy undercoat of sheep?

The correct term for the curly or wavy undercoat of sheep is wool. Wool is specifically the soft, crimped fiber that grows on sheep and other animals, which is used primarily for textile production. It provides warmth and insulation, making it an important material for clothing and other fabric products.

Fleece, while often used interchangeably with wool, typically refers to the entire coat of sheared sheep or to a type of fabric that mimics wool. The term hair refers to a different type of fiber found in animals and does not accurately describe the undercoat characteristics of sheep. Curl isn't a commonly used term in this context; it generally describes the shape of the wool but does not refer to the fiber type itself. Thus, wool is the most precise answer regarding the curly or wavy undercoat.