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1. An expression used to speculate about something. What if I was able to drive you there. Would that change anything? What if aliens did exist? What would that mean for society?
2. A speculative scenario; a speculation about an outcome that cannot yet be known or ascertained. There is a possibility that we could write off some of these losses, but it's still a big what if at this point.
Suppose that, as in What if the speaker doesn't get here in time? This expression is in effect a shortening of "what would happen if." It was first recorded about 1420.
1. What would occur if; suppose that: What if we were rich?
2. What does it matter if: What if he gets angry?—I don't care.