An eclipse is when one celestial object gets in front of the sun and casts a shadow on another object.

In a solar eclipse, the moon is between the Earth and sun, and part of the Earth falls under the moon's shadow.

In a lunar eclipse, the Earth is between the sun and the moon and casts a shadow on the moon.

It's not possible for the sun to get between the Earth and moon (because the distance between the Earth and moon is smaller than the distance from the Earth to the sun), but other planets can be on the other side of the sun, in which case we call it an occultation.