Earth or the Earth is the planet on which we live. People usually say Earth when they are referring to the planet as part of the universe, and the Earth when they are talking about the planet as the place where we live.

The earth is the land surface on which we live and move about.

Earth is the substance on the land surface of the earth, for example clay or sand, in which plants grow.

An earth is a hole in the ground in which an animal such as a fox lives.

The earth in an electric plug or piece of electrical equipment is the wire through which electricity can pass into the ground, which makes the equipment safe if something goes wrong with it.

On earth is used for emphasis in questions that begin with words such as 'how', 'why', 'what', or 'where'. It is often used to suggest that there is no obvious or easy answer to the question being asked.

On earth is used for emphasis after some negative noun groups, for example 'no reason'.

On earth is used for emphasis after a noun group that contains a superlative adjective.

If you come down to earth or back to earth, you have to face the reality of everyday life after a period of great excitement.

If you run someone or something to earth, you find them after searching for them for a long time.

If you say that something cost the earth or that you paid the earth for it, you are emphasizing that it cost a very large amount of money.