A mathematical series in which each consecutive number is the sum of the two preceding numbers: 1, 1, 2, 3, 5, 8, 13, 21, 34, 55, 89, 144, etc. named after Leonardo of Pisa, who was known as Fibonacci.
As the series progresses, the ration of a Fibonacci number divided by the immediately preceding number comes closer to 1.618, the “golden mean”, a ratio found in many natural phenomena as well as man-made objects like the Parthenon and the Great Pyramid. (The inverse, .618, has a similar significance.) Traders use various permutations of Fibonacci numbers to project retracement levels, among other things.