1
- quick to notice any unusual and potentially dangerous or difficult circumstances; vigilant警惕的, 警觉的, 机警的：
an alert police officer discovered a lorry full of explosive
一位警惕的警官发现了一辆满载炸药的卡车
schools need to be constantly alert to this problem.
各学校对此问题应该始终保持警惕。
1.1
- able to think clearly; intellectually active机灵的, 机敏的：
she remained active and alert until well into her eighties.
她80多岁时仍然活跃机敏。
mass noun
1- the state of being watchful for possible danger警惕, 警戒：
security forces were placed on alert.
安全部队处于警戒状态。
1.1
- count noun an announcement or signal warning of danger警报, 警示：
a bomb alert
轰炸警报
an alert sounded and all the fighters took off.
警报响了, 所有的战斗机随之起飞。
1.2
- count noun a period of vigilance in response to such a warning警戒期：
traffic was halted during the alert.
警戒期间交通被中断。
1.3
- count noun a signal on an electronic device that prompts the user to do something or attracts their attention（电子设备的）提示信号：
a vibrating alert is a discreet alternative to ringtones.
相对于铃声信号而言, 振动提示信号是一种谨慎的替代手段。
with obj.
- warn (someone) of a danger, threat, or problem, typically with the intention of having it avoided or dealt with（尤指为了避免或应付危险、威胁或问题而）提醒, 警告（某人）：
he alerted people to the dangers of smoking
他提醒人们注意吸烟的种种危害
police were alerted after three men drove away without paying.
三个男人没付钱就开车走掉之后, 警方开始警觉起来。
短语
on the alert
- vigilant and prepared处于戒备状态：
security forces are on the alert for an upsurge in violence.
安全部队处于戒备状态, 以防暴力升级。
派生词
alertly
adveralertness
noun词源
late 16th cent. (originally in military use)： from French alerte, from Italian all' erta 'to the watchtower'.