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To signal with one's hand for someone or something to stop or come over to one's location. A noun or pronoun can be used between "wave" and "down." I managed to wave a car down to help me fix my tire on the side of the road. We're already running late, so let's just wave down a cab.
To signal and cause someone or something to stop by waving the hand or arm: I waved down a cab. The stranded motorist waved a police car down.