Definition of 'Bollinger Band'
A band plotted two standard deviations away from a simple moving average, developed by famous technical trader John Bollinger.
Investopedia explains 'Bollinger Band'
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). The tightening of the bands is often used by technical traders as an early indication that the volatility is about to increase sharply.
This is 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.
Want to know more? Read The Basics Of Bollinger Bands