In the solar system, the inclination of the orbit of a planet is defined as the angle between the plane of the orbit of the planet, and the ecliptic, which is the orbit of Earth.
The inclination of orbits of natural satellites is measured relative to the equatorial plane of the body they orbit (the equatorial plane is the plane perpendicular to the axis of rotation of the central body). For the Moon however, the inclination is measured with respect to the ecliptic, i.e. the plane of the orbit that Earth and Moon track together around the Sun.
The inclination of a binary star is defined as the angle between the orbital plane and the direction to the observer. Binary stars with inclination close to 90 degrees are often eclipsing.
See also: axial tilt