警戒 jǐngjiè to warn
to warn; to alert; to be on the alert; to stand guard; sentinel
How to remember 警戒: 警 guard + 戒 to warn = to warn.
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How it is used
- To stand guard / keep watch, and to be on alert - 加强警戒 (jiāqiáng jǐngjiè) tighten security/watch; 警戒线 (jǐngjiè xiàn) a cordon/警戒 line.
- More external and military than 警惕 (jǐngtì, inner mental vigilance): 警戒 implies posted guards and physical watch.
- Formal register.