"2.19 Ground resolvents. If C and D are two ground clauses, and L ⊆ C, M ⊆ D are two singletons (unit sets) whose respective members form a complementary pair, then the ground clause: (C − L) ∪ (D − M) is called a ground resolvent of C and D."

Robinson, J. A. (1965), 'A Machine-Oriented Logic Based on the Resolution Principle', Journal of the ACM (JACM), v.12 n.1, p.26