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