In general, a person who provides
visual information for another persons use.
Spotters are extremely important for
artillery. In
WWI the
Germans were able to do terrible damage by using early airplanes as spotters. When the troops saw a plane pass over they would immediately brace for the coming explosion. A person who calls in an
aerial bombardment can also be considered a
spotter.
Also, a person who helps another person to exercise. This generally consists of helping the person out if their strength fails . Like stopping weights from crushing a lifter, or stopping someone from breaking a leg by falling off gymnastic equipment.