The great grammarian Henry Fowler coined this term to refer to a restrictive clause. A defining clause looks to the noun modified and singles it out among others that could exist in the context. A defining clause points a finger at the noun modified and says, “that noun, not any others named by that noun.”
A defining clause begins with the relative pronoun that and is not set off by commas. Of course, defining clauses could begin with the personal relative pronouns—who, whom, whose. The key, then, becomes the absence of any comma setting off the clause.
Here are some defining clauses:
The judge who wrote the opinion assumed the bench in 1999.
The novel that climbed to the top of the charts set a sales record.
Here's the book I told you about. (This clause has its own independent subject “I,” so you may drop the that.)
See nondefining clause.