When an astronomical body moves into the shadow of another astronomical body.
A Solar Eclipse is when the Moon moves between the Sun and the Earth. In art, this will look like a glowing ring around a dark circle.
A Lunar Eclipse is when the Moon is in the Earth's shadow. In art, this will look like a red tinted moon, with a bit of a gradient.
Other types of eclipses exist, but are not likely to be relevant for tagging.