Learner's definition of LOOKOUT
a person who watches an area and warns others if there is danger
a high place or structure from which you can see a wide area
be someone's lookout
— used to say that someone who makes a bad decision or does something foolish is responsible for the result and will have to deal with it alone
keep a lookout for
to watch for (something or someone)
on the lookout for
looking or searching for (something or someone)
An avid shopper, she was always on the lookout for sales.
The police are on the lookout for two men who escaped from custody yesterday.