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.