Spotter (noun)

1. A person who searches for something.
2. A person who watches and reports something.
3. A person who supervises weightlifting to ensure safety.

Origin:

Mid 19th century: from spot + -er.

Examples:

1. The spotter was looking for rare birds.
2. The spotter reported enemy movements to his commander.
3. The spotter watched the weightlifter to ensure proper form.
4. The spotter was an expert in his field.
5. The spotter was always on the lookout for new discoveries.