Orbital inclination refers to the angle between the orbital plane of a planet and a reference plane, typically the plane of the celestial equator or the ecliptic plane. It measures how tilted or inclined the planet's orbit is relative to the reference plane.

In our solar system, most planets orbit the Sun in a relatively flat plane called the ecliptic plane. This plane is defined by the Earth's orbit around the Sun. The reference plane for measuring orbital inclination is often taken as the ecliptic plane, which is also the plane of Earth's orbit around the Sun.

The orbital inclination of a planet is determined by factors such as its formation history, gravitational interactions with other celestial objects, and the dynamics of the early solar system. As a result, each planet can have a different orbital inclination.

If a planet has an orbital inclination of 0 degrees, it means its orbit lies in the same plane as the reference plane. This is often referred to as a "zero inclination orbit" or an orbit that is "in the plane of the ecliptic." On the other hand, if a planet has an orbital inclination of 90 degrees, its orbit would be perpendicular to the reference plane, making it highly inclined or even polar.