Correlators in English grammar are function words that are the first word or words in pairs of conjunctions that also join or link two or more words, phrases, and clauses. Function words perform definite grammatical functions but lack definite lexical meaning.
Correlator is a grammatical function.
The grammatical form that can function as the correlator in English grammar is the correlating conjunction.
Some correlating conjunctions in English are both, either, neither, not only, and whether.
Brinton, Laurel J. & Donna M. Brinton. 2010. The linguistic structure of Modern English, 2nd edn. Amsterdam: John Benjamins Publishing Company.
Hopper, Paul J. 1999. A short course in grammar. New York: W. W. Norton & Company.
Huddleston, Rodney. 1984. Introduction to the grammar of English. Cambridge: Cambridge University Press.