dipole

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.

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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.

The length of so-called half-wavelength or λ/2 Dipole antennas are, in reality, typically less than λ/2. The input impedance Z_in of a dipole exactly λ/2 long is 73 + j42.5 Ohms. To achieve resonance, the dipole is typically shortened to around .47-.48 λ until the reactive part of the impedance vanishes. The thicker the wire or tubing the dipole is made out of, the shorter the dipole will need to be to be resonant. This is, of course, assuming that the dipole is ideal, i.e. it's perfectly straight and not suspended over some sort of conducting ground (which most are if you live on earth).

Dipoles are not necessarily limited in length to λ/2. In fact, dipoles with lengths of several wavelengths can have very useful radiation patterns and are often used in practice. For example, there are shortwave radio stations that broadcast on multiple frequencies simultaneously using the same dipole antenna (using duplexers, or clever use of tuning stubs). The radiation pattern will be different for each of the transmitted frequencies.