The word "unless" is a conjunction used to introduce a conditional clause. It means "except under the circumstances that" or "if not." It sets a condition that must be met for something else to happen or be true. For example, if you say, "I won't go to the party unless you come," it means you will only go to the party if the other person agrees to come along.
The word "unless" is derived from the Old English phrase "unles," which combines the negative prefix "un-" with "les," meaning "less." The term evolved over time into its current form, retaining the original implication of negating a condition.