More specifically, it is called a dependent clause or a subordinate clause. Here's why:
* Phrase: A group of related words that does not have a subject and verb.
* Clause: A group of related words that contains a subject and verb.
* Dependent Clause: A clause that cannot stand alone as a complete sentence. It depends on an independent clause to make sense.
Example:
* Because the cat is sleeping (dependent clause)
* The cat is sleeping (independent clause)
Complete Sentence: Because the cat is sleeping, I will be quiet.