Something that cannot be done, achieved, or exist because it goes against the laws of nature or reason.
From Latin im- (not) + possibilis (possible). The word entered English in the 14th century to describe things that defy possibility according to the understanding of physics or logic.
What was 'impossible' has changed wildly throughout history—flying was impossible until 1903, and splitting the atom was considered impossible until 1942. Today's impossibilities might be tomorrow's everyday tech, which shows how much our sense of what's possible depends on current knowledge.
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