Final answer:
A group of two or more words that cannot stand on its own as a complete sentence is called a phrase. Subordinate clauses are a type of phrase that express less important ideas and cannot stand alone as a sentence.
Step-by-step explanation:
A group of two or more words that cannot stand on its own as a complete sentence is called a phrase. A phrase can be any group of words that is missing either a subject or a verb. Subordinate clauses are a type of phrase that begin with subordinating words or phrases like although, because, if, when, since, and so on. These subordinate clauses express less important ideas and cannot stand alone as a sentence.