Here's why:

* Terrestrial planets are rocky planets. They are primarily composed of silicate minerals and metals. The moon, while rocky, is significantly different in composition than the Earth.

* Terrestrial planets are large enough to have cleared their orbital paths of other objects. The moon orbits the Earth and is not considered to have cleared its path.

* Terrestrial planets are defined by their formation process. The moon formed differently than the terrestrial planets, likely from a collision with Earth.

The moon is classified as a natural satellite or a moon.