"the secondary term of a relation, that to which something is related," 1640s, perhaps a back-formation from correlation or from correlate (adj.), from a Medieval Latin adjectival use of the Latin past participle.
1742, "to be reciprocally related" (intransitive), back-formation from correlation, or else a verbal use of the noun. Transitive sense of "to place in reciprocal relation" is by 1849. Related: Correlated; correlating; correlative.