The word "unless" is a conjunction used to introduce a condition that must be met for something to happen. It indicates that one event will not occur if another event does not take place. For example, in the sentence "I won't go to the party unless you invite me," the invitation is a necessary condition for attending the party. In logic and mathematics, "unless" can be understood as a way to express a conditional relationship. It often implies a negative condition, meaning that if the specified condition is not fulfilled, the opposite outcome will occur. This can be seen in statements like "You won't succeed unless you work hard," where hard work is essential for success.