In general, a perturbation is a slight nudge that, temporarily or permanently, displaces an object or system out of equilibrium.
In astronomy, a perturbation is a change in the orbit of a body, usually as a result of the gravitational effect of another, typically much larger, body. The planets mutually perturb one another; the orbits of satellites are perturbed by their mutual interactions and also by the Sun. Most dramatically, the giant outer planets, especially Jupiter, can radically alter the orbits of asteroids and comets and sending them careening on new paths into the inner solar system.
In spaceflight, a perturbation is any disturbance to a planned trajectory or orbit, caused by atmospheric drag, gravitation, radiation pressure or some other external influence.