The ratio between the orbital periods of two bodies. Expressed as a dimensionless ratio > 1. Alternately: The inverse of the ratio the orbital frequencies. Whole number orbital resonances with large bodies are often unstable, owing to undue gravitational influences. In the asteroid belt there are gaps corresponding to certain whole number orbital resonances with Jupiter: i.e. 2:1, 3:1, 5:2, 7:2. The Cassini gap appears in the rings of Saturn, produced by an orbital resonances of 2:1 with the inner moon Mimas. As Grzcyrgba points out, the nearly 2:1 orbital resonance between Europa and Io is responsible for the tremendous tidal forces that cause Io's volcanism.