1. A small knobby pair of modified wings in some two-winged insects, used to maintain equilibrium during flight.
2. In ancient Greek sports, a pair of handheld weights used during long jump.
There is an excellent photo of insect halteres on a crane fly at wikipedia, along with a fine explanation of their source, and their function as gyroscopes.
Etymology of the entomology term: 18th century, from the Greek term for the long jump weights.