Wiktionary
n. (context astronomy English) The surface (plane) that contains an orbit.
WordNet
n. (astronomy) the plane on which a body is orbiting
Wikipedia
The orbital plane of an object orbiting another is the geometrical plane in which the orbit lies. The orbital plane is defined by two parameters, inclination (i) and longitude of the ascending node (Ω). Three non- collinear points in space suffice to determine the orbital plane. 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.
By definition the inclination of a planet in the solar system is the angle between its orbital plane and the orbital plane of the Earth (the ecliptic). In other cases, for instance a moon 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.
Usage examples of "orbital plane".
It had only fired the vernier thrusters, not the rocket motors that would move it into an altogether different orbital plane.
Its height above the orbital plane represented its component in ordinary 3-space, and as it was jostled further and further from its original direction, the neutron stars began to spiral together.
Captive Suns Seven to Eleven were at varying distances from the Sphere, but they were spaced exactly ninety degrees apart from each other, and shared an orbital plane forty-five degrees away from the Sunstar’.
It was no holotank, but two dimensions were all that was required to display the orbital plane of this system's cold, worthless planets.
The distance was beyond the incidental reach of most of the sensors of the planet's geosync satellite, which were faced in toward the planet anyway, and outside the observation cone of the outward facing sensors, which were aimed perpendicular to the orbital plane, the customary direction for starships to approach a planetary system.
Gargantua, the remaining gas-giant planet of the system, moved in the same orbital plane, and it, too, had its own point of close approach to Mandel.