The average true range (ATR) is a technical analysis indicator that breaks down the full price range of an asset for a certain period to determine market volatility. J. Welles Wilder Jr. first discussed it in his book New Concepts in Technical Trading Systems.

The largest of the current high less the current low, the absolute value of the current high less the previous close, and the absolute value of the current low less the previous close is considered to be the actual range indication.
The ATR is then a moving average of the real ranges, often using 14 days.