| 1. | fully aware and attentive; wide-awake; keen: an alert mind. |
| 2. | swift; agile; nimble. |
| 3. | an attitude of vigilance, readiness, or caution, as before an expected attack. |
| 4. | a warning or alarm of an impending military attack, a storm, etc.: We'd just boarded the bus when the alert sounded. |
| 5. | the period during which such a warning or alarm is in effect. |
| 6. | to warn (troops, ships, etc.) to prepare for action. |
| 7. | to warn of an impending raid, attack, storm, etc.: The radio alerted coastal residents to prepare for the hurricane. |
| 8. | to advise or warn; cause to be on guard: to alert gardeners to the dangers of some pesticides. |
| 9. | on the alert, on guard against danger; in readiness; vigilant: The state police are on the alert for an escaped convict believed to be in the area. |

a·lert (ə-lûrt') adj.
To notify of approaching danger or action; warn: a flashing red light that alerted motorists to trouble ahead. [French alerte, from Italian all' erta, on the lookout : alla, to the, on the (from Latin ad illam, to that : ad, to; see ad- + illam, feminine accusative sing. of ille, that, the; see al-1 in Indo-European roots) + erta, lookout (from past participle of ergere, to raise, from Latin ērigere; see erect).] a·lert'ly adv., a·lert'ness n. |
on the alert
Watchful, fully prepared, as in The inspectors are always on the alert for a manufacturing error. [Late 1700s]