驾驶员 is the formal/official term — used on signs, licenses, and in regulations. In everyday speech, 司机 (sījī) is far more common for 'driver.' 驾驶员 can also mean 'pilot' when paired with 飞机.
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Formal/official. You'll see 驾驶员 on traffic signs and legal documents. In conversation, use 司机 instead.
