An orbital resonance is a stable configuration of the orbits of two bodies orbiting the same third, such that they follow a pattern, e.g., their orbital periods being related by two small integers. Kinematically, such an orbital relationship is termed commensurability, which can develop when the forces of gravity while the two are relatively close to each other tend to draw them toward it, and can remain stable when gravity provides negative feedback to any move away from it. Such a resonance, with a simple integer ratio between the periods is called a mean motion resonance (MMR). The orbital periods of three of Jupiter's moons form such ratios and the orbital periods of Pluto and Neptune have a ratio of 3:2.
A mean motion resonance tends to increase the eccentricity of the orbits, and in some cases can eventually similarly increase their orbital inclination (between their orbital planes), in which case it is termed an inclination-type resonance (or inclination resonance). Resonances only affecting eccentricity are termed eccentricity-type resonances (or eccentricity resonances).
A secular resonance is a resonance of orbits not on each circuit, but on a longer-term (secular) pattern. It might show up as a pattern in the orbits' precessions. The term is common in analysis of the orbits of asteroids and other minor planets.