1. Any of the several large, dark plains on the moon and Mars.

A reference to one of the large dark expanses of basalt on the moon and Mars, many of which fill impact basis; for example,

• Mare Nubium, "Sea of Clouds".
• Mare Serenitatis, "Sea of Serenity".
• Mare Tranquillitatis, "Sea of Tranquility".

So named because Galileo believed that the lunar features were seas when he first saw them through a telescope.
2. Etymology: from Latin mare, "sea".

3. A fully mature female horse or other equine animal: Little Susi was finally old enough to ride on her mare!
4. Etymology: from Middle English mere, mare; from Old Saxon mere to Old English mearh, "horse"; so, it is obvious that this mare is not related to the other mare referring to the "sea".