Dictionary definition · alert
noun
- condition of heightened watchfulness or preparation for action"bombers were put on alert during the crisis"
- a warning serves to make you more alert to danger
- an automatic signal (usually a sound) warning of danger
verb
- warn or arouse to a sense of danger or call to a state of preparedness"The empty house alarmed him"
adjective
- engaged in or accustomed to close observation"caught by a couple of alert cops"
- quick and energetic"a brisk walk in the park"
- mentally perceptive and responsive"an alert mind"
Source: Open English WordNet (CC BY 4.0).