1. 警觉的；警惕的；留神的；机敏的
    If you are alert, you are paying full attention to things around you and are able to deal with anything that might happen.

    e.g. We all have to stay alert...
           我们大家都必须保持警惕。
    e.g. He had been spotted by an alert neighbour.
           一个警觉的邻居发现了他。

2. 充分意识到…的;完全感觉到…的
    If you are alert to something, you are fully aware of it.

    e.g. The bank is alert to the danger.
           银行充分意识到了危险。

3. 警戒（状态）；戒备（状态）
    An alert is a situation in which people prepare themselves for something dangerous that might happen soon.

    e.g. Due to a security alert, this train will not be stopping at Oxford Circus.
           由于安全戒备的原因，该次列车在“牛津街口广场”站将不停留。

4. 向…报警；使警觉；使警惕；使注意
    If you alert someone to a situation, especially a dangerous or unpleasant situation, you tell them about it.

    e.g. He wanted to alert people to the activities of the group...
           他想提醒人们小心该团体的活动。
    e.g. I was hoping he'd alert the police.
           我当时希望他会报警。

5. see also: red alert

6. 处于警戒状态;保持戒备
    If soldiers or police are on alert, they are ready to deal with anything that may happen.

    e.g. Soldiers and police have been put on alert.
           士兵和警方都已进入警戒状态。

7. 准备就绪；防备着
    If you are on the alert for something, you are ready to deal with it if it happens.

    e.g. They want to be on the alert for similar buying opportunities.
           他们希望随时掌握有关类似购买机会的信息。