In linguistics, a copula is a word used to link the subject of a sentence with a predicate (a subject complement or an adverbial). Although it might not itself express an action or condition, it serves to equate (or associate) the subject with the predicate. The word 'copula' originates from the Latin noun for a "link or tie" that connects two different things (for a short history of the copula see the appendix to Moro 1997 and references cited there).
a statistical tool describing how the distribution of single risks join together to form joint risk distribution. In the case of portfolio credit risk, copulas describe how patterns of individual default risk join together to form the distribution of loss on a portfolio