Bollinger Band
A band plotted two standard deviations away from a simple moving average. 
Above is an example of bollinger bands, the price of the stock is banded by an upper and lower band along with a 20-day simple moving average.
Investopedia Commentary
Because standard deviation is a measure of volatility, Bollinger Bands adjust themselves to the market conditions. When the markets become more volatile the bands widen (move further away from the average), and, during less volatile periods, the bands contract (move closer to the average).
This is among one of the most popular technical analysis techniques. The closer the prices move to the upper band, the more overbought the market, and the closer the prices move to the lower band, the more oversold the market.
Related Links
Trading Trend Or Range?
The Basics of Bollinger Bands
Profiting From The Squeeze
Using Bollinger Band "Bands" to Trade Trend in FX
See also: Moving Average, Standard Deviation, Technical Analysis, Trend
Also spelled: bolinger