Orbital resonance is a stable configuration of the orbits of two bodies orbiting the same third, such that their orbital periods are 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.
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. Such a resonance, where there is a simple integer ratio between the periods is called a mean motion resonance (MMR).
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.