Copula

noun

1. (grammar) A word, usually a verb, used to link the subject of a sentence with a predicate (usually a subject complement or an adverbial), that unites or associates the subject with the predicate.
2. A function that represents the association between two or more variables, independent of the individual marginal distributions of the variables.
3. A device that connects two or more keyboards of an organ.
4. The act of copulation; mating.