The word "if" is a conjunction used to introduce a conditional clause. It sets up a situation where one event depends on another. For example, in the sentence "If it rains, we will stay indoors," the action of staying indoors is contingent upon the condition of rain. In logic and mathematics, "if" is often used to formulate statements and hypotheses. It helps in creating "if-then" statements, which express a cause-and-effect relationship. For instance, "If A is true, then B must also be true" illustrates how one condition leads to another outcome.