An electronic device worn on the body to aide in quickly finding buried avalanche victims. Also called an avalanche beacon, it has the ability to send and receive a 457khz radio signal.
How beacons work:

Beacons are simply electronic devices about the size of a large mobile phone that both transmits and receives an electronic signal. Everyone in the party wears one and each member turns it on when they leave the house or leave the car to head into the backcountry. (Wear them UNDER your jacket to keep the batteries warm, which also helps keep it from being torn off your body during the ride.) When turned on, the beacon transmits an electronic “beep” about once per second. Then, if someone is buried, everyone else in the party turns their beacon to receive, and they can hear the signal from the buried victim's beacon; the signal gets stronger the closer you get. The range of most beacons varies between 40 and 80 meters depending on the brand. And yes, all beacons work on the same international standard frequency.

Caveat: Beacons only work if you practice regularly with them and most people don’t practice enough. As a result, beacon use has not increased survivability rates as much as one would hope. For people who practice regularly, however, beacons have saved many lives and they work very well. In addition, about a quarter of avalanche victims die from hitting trees and rocks on the way down, so beacons can only help the other three quarters who survive the ride before getting buried.

Rap on analog vs. digitalbeacons

Back in the old-days, all beacons were analog, but with the rise of computer technology, most manufacturers use computer chips to filter, process and display the signal, which makes finding a buried beacon faster, simpler and more idiot-proof. You can now find a continuum between all-analog and all-digital beacons with a variety of combinations in between. Contrary to popular belief, the so-called "digital" beacons still use an analog signal like the other beacons, but they simply use a microprocessor to filter, compute and display the signal. And finally, digital does not necessarily mean better in all cases. For instance, some professional avalanche teams still use all-analog beacons because they tend to have a longer range and the analog audio can be an advantage in multiple burials. But especially for the casual user and people who don’t practice very often with their beacons, the digital technology has made beacon searches simpler, easier and faster.

The technology of beacons changes so rapidly that anything we say here would be quickly out of date, so be sure to read the latest reviews of beacons in the magazines and web sites. Talk to the salespeople in the stores and be sure to shop around and play with several different models. There is no “best” beacon on the market, just advantages and disadvantages with each brand and model.

Practice, Practice, Practice

No matter what beacon you buy, the most important step is to practice, practice, practice. Remember that finding a single beacon in a parking lot is far easier than finding multiple buried beacons in a realistic situation, especially when a loved one is under the snow. Many mountain locations now have automated or semi automated beacon trained centers. These allow one to practice both single or multiple victim rescues, solo or as a group. Check with your local Avalanche Forecast Center for a beacon training facility near you.

Additional Terms:
Anchors Hard Slab Avalanche Slide
Aspect High Danger Sluff
Avalanche High Marking Snowpit
Avalanche Path Isothermal Soft Slab Avalanche
Avalanche Transceiver Layer, Snow Stability
Bed Surface Leeward Stability Test
Collapse Loading Starting Zone
Concave Slope Loose Snow Avalanche Stepping Down
Considerable Danger Low Avalanche Hazard Sun Crust
Convex Slope Melt-Freeze Snow Surface Hoar
Cornice Metamorphism, Snow Sympathetic Trigger
Corn Snow Moderate Danger Temperature Gradient
Couloir Persistent Weak Layers Terrain Trap
Cross Loading Point-Release Track
Crown Face Probe Trigger
Danger Ratings Propagation Trigger Point
Deep Slab Avalanche Rain Crust Upside-Down Storm
Density, Snow Remote Trigger Weak Layer
Depth Hoar Rime Weak Interface
Dry Snow Avalanche Runout Zone Wet Snow Avalanche
Extreme Danger Sastrugi Windward
Faceted Snow Settlement Wind Loading
Fracture Ski or Slope Cut Wind Slab
Glide Skinning, Skin Track Whumpf
Graupel Slab