|Definition - A conjunction that is used to express a condition — e.g. if, since, and unless.|
Example - If he goes, I will stay.
(In the above, if is the conjunction that introduces the condition if he goes.)
Oxford English Dictionary - The term's first citation is from 1786:
"Those words which are called conditional conjunctions, are to be accounted for in all languages … as I have accounted for If and An."
(H. Tooke Purley 56)