What is a non-defining relative clause?

“Non-defining relative clause” is another name for a nonrestrictive clause, which is a type of relative clause (or adjective clause) that gives extra, nonessential information about the noun or pronoun it modifies (e.g., “I am going to visit Lake Superior, which is the world’s largest freshwater lake”).

Nonrestrictive clauses can be removed from a sentence without changing its meaning.