Spotter

In general, a spotter is someone trained to look for or "spot" something.

Weather

In particular, skywarn spotters are frequently used in the U.S. to report severe weather. These volunteers are trained by the National Weather Service, and given a phone number or amateur radio frequency to call if a major weather event, such as a tornado or severe thunderstorm occurs where the spotter is located.

Weightlifting

In weightlifting, a spotter is a person who stands by the person who is lifting the weights, in order to help the person should he or she be unable to lift the weight off of his or her chest.