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What is the difference between “par hasard” and “au hasard”?

Le hasard is chance, and two everyday phrases split it in half. Par hasard means by coincidence: je l’ai vu par hasard, “I ran into him by chance.” Au hasard means without a plan: choisis une carte au hasard, “pick a card at random.” Swap the preposition and you swap luck for randomness — a distinction French keeps carefully.

The word is usually traced back through Spanish azar to an Arabic word for a die, which is why les jeux de hasard — literally games of chance — is the standard French term for gambling. That dice origin is the common account rather than a settled one; some dictionaries prefer a different Arabic source. Either way, the meaning travelled from “a throw of the dice” to “chance” itself.