alert

nouns

condition of heightened watchfulness or preparation for action
"bombers were put on alert during the crisis"
alert, qui vive
a warning serves to make you more alert to danger
alert, alerting
an automatic signal (usually a sound) warning of danger
alarm, alarum, alert, warning signal

verbs

warn or arouse to a sense of danger or call to a state of preparedness
"The empty house alarmed him"; "We alerted the new neighbors to the high rate of burglaries"
alarm, alert

adjectives

engaged in or accustomed to close observation
"caught by a couple of alert cops"; "alert enough to spot the opportunity when it came"; "constantly alert and vigilant, like a sentinel on duty"
alert, watchful
is derived from
alertness
antonym
unalert
quick and energetic
"a brisk walk in the park"; "a lively gait"; "a merry chase"; "traveling at a rattling rate"; "a snappy pace"; "a spanking breeze"
alert, brisk, lively, merry, rattling, snappy, spanking, zippy
mentally perceptive and responsive;"an alert mind"
"alert to the problems"; "alive to what is going on"; "awake to the dangers of her situation"; "was now awake to the reality of his predicament"
alert, alive, awake
is derived from
alertness