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Fast definition: moving or able to move, operate, function, or take effect quickly; quick; swift; rapid: a fast pain reliever;. See examples of FAST used in a sentence.

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Grammar Fast, quick or quickly? Fast and quick mean moving with great speed. Fast is both an adjective and an adverb. Quick is an adjective and the adverb form is quickly. …

Fast and rapid are often used interchangeably, though fast is more often applied to the person or thing in motion, and rapid to the activity or movement involved: a fast runner; rapid strides.

You use fast to say that something happens without any delay. When you've got a crisis like this you need professional help–fast! We'd appreciate your leaving as fast as possible. Fast is also …