a relative clause that describes or supplements but is not essential in establishing the identity of the antecedent and is usually set off by commas in English. In This year, which has been dry, is bad for crops the clause which has been dry is a nonrestrictive clause. Also called descriptive clause. Cf. restrictive clause.
Random House Unabridged Dictionary, Copyright © 1997, by Random House, Inc., on Infoplease.