Definition of "Alert" :
noun: an automatic signal (usually a sound) warning of danger
noun: a warning serves to make you more alert to danger
noun: condition of heightened watchfulness or preparation for action
"Bombers were put on alert during the crisis."
verb: warn or arouse to a sense of danger or call to a state of preparedness
"We alerted the new neighbors to the high rate of burglaries."
adjective: engaged in or accustomed to close observation
"Caught by a couple of alert cops."
adjective: mentally perceptive and responsive
"An alert mind."
adjective: quick and energetic