copula vs linking verb

copula

noun
  • The act of copulation; mating. 

  • 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. 

  • A device that connects two or more keyboards of an organ. 

  • A function that represents the association between two or more variables, independent of the individual marginal distributions of the variables. 

linking verb

noun
  • Copula, copulative verb. 

How often have the words copula and linking verb occurred in a corpus of books? (source: Google Ngram Viewer )