Definition of ALERT

paying close attention usually for the purpose of anticipating approaching danger or opportunity <she needed to stay alert throughout the train ride so as not to miss her stop>
Phrases on guard, on one's toes, on the alert, on the ball, on the qui vive, on tiptoe
Antonyms asleep
having or showing quickness of mind <an alert and well-trained sheep dog who can handle difficult situations>
having or showing the ability to respond without delay or hesitation <an alert force of commandos, ready to go on a mission at a moment's notice>

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