A dipole is a system which has an overall neutral charge but where the electrostatic charges are so distributed such that one area is more negative and another more positive. The strength of this effect, and so the dipole, is referred to as the dipole moment. An item that has a dipole is referred to as polar.

The water molecule has a significant dipole as the oxygen molecule is significantly more negative than the hydrogens and they are not all in a line. If the water molecule were linear (like carbon dioxide) then it would not have an overall dipole. This is important as the semi-ionic nature of water is why it is a liquid at room temperature.

Dipole is also the name for a particular form of aerial involving two rods which are almost touching, arranged like a single rod with a small piece removed from the middle.