a subordinate clause introduced by a relative pronoun, adjective, or adverb, either expressed or deleted, esp. such a clause modifying an antecedent, as who saw you in He's the man who saw you or (that) I wrote in Here's the letter (that) I wrote. Cf. definite relative clause, indefinite relative clause.
Random House Unabridged Dictionary, Copyright © 1997, by Random House, Inc., on Infoplease.