A copula is a form that indicates a stative link between the two argument positions of an °equational, °ascriptive or °locational clause.
The definition is from Haspelmath (2025: 17).
copula (STR) = a strategy in which an additional morpheme is used in nonprototypical predication. Copulas can be divided into verbal copulas and nonverbal copulas. (Sections 1.4, 10.2)