The word "if" is a conjunction used to introduce a conditional clause. It sets up a situation that may or may not happen, indicating a possibility. For example, in the sentence "If it rains, we will stay indoors," the outcome depends on the condition of rain. In logic and mathematics, "if" is often used to express hypotheses or propositions. It helps in forming statements that explore relationships between different variables. For instance, "If A is true, then B must also be true" illustrates a cause-and-effect relationship that can be analyzed further.