The great grammarian Henry Fowler coined this term to refer to a nonrestrictive clause. A nondefining clause looks to the noun modified and adds information about it. It does not single it out among others that could exist in the context. Instead, the identity of the modified noun either is already known from context or is known because there's just one of those things named by the noun.
You should introduce nondefining clauses with the relative pronoun which (unless who, whom, or whose is appropriate). Commas must set off the clause from the rest of the sentence.
Following are some examples of nondefining clauses. It is assumed that, from context, the reader would know the identity of the thing, person, or idea named by the noun. The clause is not necessary for identification. It is, in a word, nondefining.
My best friend, who lives next door, came to dinner. The U.S. Supreme Court, which is across the street from Congress, decided the case. The book, which arrived in the mail, enraptured the entire family.
See defining clause.