Noun

1. An optical instrument for aiding the eye in aiming, as on a firearm or surveying instrument
2. An instance of visual perception
3. Anything that is seen
4. The ability to see; the visual faculty
5. A range of mental vision
6. The range of vision
7. The act of looking or seeing or observing
8. (often followed by `of') a large number or amount or extent

Verb

1. Catch sight of; to perceive with the eyes
2. Take aim by looking through the sights of a gun (or other device)