The moon is the object that you can often see in the sky at night. It goes round the Earth once every four weeks, and as it does so its appearance changes from a circle to part of a circle.

A moon is an object similar to a small planet that travels around a planet.

If you are mooning around, you are spending time doing nothing in particular, for example because you feel unhappy or lazy, or are worried about something.

If you moon at someone, you turn your back to them and show them your bare bottom.

If you say that something happens once in a blue moon, you are emphasizing that it does not happen very often at all.

If you say that you are over the moon, you mean that you are very pleased about something.