watch

1 [countable] a small clock that you wear on your wrist or keep in your pocket:

My watch has stopped.

look at/glance at/consult your watch

She glanced nervously at her watch.

• How do you keep track of time if you don't wear a watch?

2 [singular, uncountable] when you watch someone or something carefully, or pay careful attention to them, so that you are ready to act if necessary

The police arrived to keep watch on the mouth of the tunnel.

watch on/over

He maintained a 24-hour watch over his son.

Security forces kept a close watch on our activities.

3

 keep a watch out for somebody/something

also be on the watch for somebody/something to be looking and waiting for something that might happen or someone you might see, especially so that you can avoid danger, trouble etc:

Be on the watch for anything suspicious.

4 [countable] a group of people whose job is to guard or protect someone or something:

We were arrested and held until the arrival of the night watch(=people responsible for keeping the streets safe at night, especially in past times).

➔ neighbourhood watch

5 [uncountable and countable] a period of time when it is someone's duty to stay somewhere and look for signs of danger:

The first watch is from now until midnight.

on watch

Who's on watch tonight?

Definition from the Longman Dictionary of Contemporary English
Advanced Learner's Dictionary.