Gann Retracement, Definition
W.D. Gann discovered that retracements in the securities he was analyzing often continued to the 50% level of the overall move before resuming in the direction of the trend. Gann concluded that this halfway level is the best point to re-enter an existing trend. 50% is also one of the main fibonacci retracement levels, along with 38.2% and 61.8%.
See also: Double Top, Double Bottom, W.D. Gann