Real Dictionary

subordinating conjunction
countable noun: A subordinating conjunction is a word such as 'although', 'because', or 'when' which begins a subordinate clause. Compare co-ordinating conjunction.

subordinating conjunction in American English
a conjunction that connects subordinate words, phrases, or clauses to some other sentence element (Ex.: if, as, so, unless, although, when)
noun: a conjunction introducing a subordinate clause, as when in They were glad when I finished

subordinating conjunction in British English
noun: a conjunction that introduces subordinate clauses, such as if, because, although, and until