The name of a popular directional antenna type. Also known as Yagi-Uda: I read that it was invented by H. Yagi and S. Uda at Tohoku Imperial University -now Tohoku University- in Japan in 1926 and it was reported in Japanese in 1926 and 1927 by Uda, while Yagi wrote the described it in English in 1928.

The Yagi antenna is an ingenious derivative of the simple half-wave dipole.

A Yagi antenna is shaped more or less like this (top view):

1      -+-
2     --+--
3    ---+---
4   ----+----

The long vertical rod (which in real life normally lies parallel to the ground) is insulated from the horizontal elements. Element 3 is the one to which the line actually attaches (also known as the driven element) and it is simply a half-wave dipole, element 4 (slightly longer than element 3) is known as the reflector, and elements 1 and 2 (slightly shorter than element 3) are known as directors - there can be more than one, some antennas have over ten. The antenna radiates (and receives preferentially) in the direction of the director - in this case it would radiate towards the top of the page.

The spacing between the elements is between 0.2 and 0.4 wavelengths, generally increasing towards the director end of the antenna. The Yagi antenna is a classic TV and Amateur Radio antenna.

Its main defect is that, at low frequencies, it becomes absolutely enormous and unwieldly. For example, if you were to build a Yagi antenna for the CB frequencies (27 MHz, that's to say about 33 feet wavelength), you would have a driven element about 16 feet wide, a reflector perhaps 17 feet wide and at least one director, about 15 feet wide, mounted on a horizontal boom about 12 feet long. Notice that the whole contraption must be stiff enough to survive windstorm, and it has to be mounted on a mast with a rotor.

Can it be done? Of course, OMs are insane, and they have proven that it can be done, but this does not mean that it will look good on top of your house.
Anyway, at higher frequencies (like 144 MHz) and higher, building Yagis becomes much easier, even with a large amount of directors. At frequences even higher, like microwaves, you get better performance with dish antennas.