The orbital plane of an object orbiting another is the geometrical plane in which the orbit lies. A common example would be: the center of the heavier object, the center of the orbiting object and the center of the orbiting object at some later time.
The orbital plane is defined in relation to a reference plane by two parameters, inclination (i) and longitude of the ascending node (Ω). Three non-collinear points in space suffice to determine the orbital plane.
By definition, the reference plane for the solar system is usually considered to be the earth's orbital plane. This defines the ecliptic, which is the path in the sky of the sun as seen from the earth.
In other cases, for instance a moon or satellite orbiting another planet, it is convenient to define the inclination of the moon's orbit as the angle between its orbital plane and the planet's equator.
For launch vehicles and artificial satellites, the orbital plane is a defining parameter of an orbit; as in general, it will take a very large amount of propellant to change the orbital plane of an object. Other parameters, such as the orbital period, the eccentricity of the orbit and the phase of the orbit are more easily changed by propulsion systems.
Orbital planes of satellites are perturbed by the non-spherical nature of the Earth's gravity. This causes the orbital plane of the satellite's orbit to slowly rotate around the Earth, depending on the angle the plane makes with the Earth's equator. For planes that are at a critical angle this can mean that the plane will track the Sun around the Earth, forming a Sun-synchronous orbit.
A launch vehicle's launch window is usually determined by the times when the target orbital plane intersects the launch site.