Etymology Dates back at least to Old English hwæt ġif, of the same meaning. A similar expression occurs in Latin in the Vulgate, Acts 23:9, "Factus est autem clamor magnus. Et surgentes quidam pharisaeorum, pugnabant, decentes: Nihil mali inveniumus in homine isto: quid si Spiritus locutus est ei, aut angelus?" ("And there arose a great cry. And some of the Pharisees rising up, strove, saying: We find no evil in this man. What if a spirit hath spoken to him, or an angel?" Douay-Rheims translation)
1
Used to introduce a suggestion or proposal for a future event.
What if we arrange it for 9:00 instead of 10:00?
2
Used to introduce a speculation about a future event.
What if we could learn to get along? What would happen then?
3
Used to introduce a speculation about a past event. Often the basis for counterfactual historical interpretations.
What if Abraham Lincoln had not been assassinated? How might the Reconstruction of the Southern States have been handled?