[mikemol]

Area in the spectrum normally reserved for television, but not intentionally actively used by television broadcasters.

Simplified (it used to work this way, now it's slightly more complicated), each television channel frequency required a certain small range (the tuner frequency +/- a certain number of MHz) to carry its signal. Ideally, the affected range of spectrum wouldn't exceed that specced amount, but electronics aren't perfect, and RF is a tricky matter.

In practice, you tend to see splatter near a particular channel, outside its allotted frequency range. Furthermore, receivers have to filter out any signal not within the frequency range for the channel they want to watch, but the filtering circuitry (called a bandpass filter) will tend to cause distortions within the intended frequency range the better they are at filtering frequencies outside that range. There are also other complicating factors, like signal overloading receivers, and mis-tuned transmitters.

This resulted in a margin space between channels, so that adjacent channels would interfere with each other less. As receiver electronics have gotten better, more selective and more error tolerant, these margin spaces became less necessary, and we have space in the spectrum that's no longer required for its original purpose of providing a margin.

That space is your 'whitespace'.

[InclinedPlane]

" As receiver electronics have gotten better, more selective and more error tolerant, these margin spaces became less necessary..."

More to the point, as the technology used to broadcast has completely changed from analog to digital the margin spaces have become completely unnecessary. Additionally, digital television does not use the entire spectrum previously allocated to analog broadcasts.

Edit: additionally, in locations where there aren't local television broadcasts on certain channels those channels can be used for wifi.

[mikemol]

Digital signal transmission tends to include an increase in error tolerance, which I mentioned. The margin reduces the strain on the error tolerance, which is also needed to cope with signal loss issues.

So, no, the margin spaces aren't completely unnecessary with digital signaling.

Furthermore, I'll pick a couple nits and point out that it's only television broadcasting which has completely switched to digital in the US. AM/FM radio is still analog by definition, and there are large swaths of spectrum which use those and other signaling methods.