The closed plane curve that crosses itself once and consists of one lobe on each side of the intersection. It can be viewed as a circle with a half twist. The fundamental group of the figure eight is the free group on two elements 
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Figure Eight
See also
Circle, Devil's Curve, Eight Surface, Figure Eight Knot, Free Group, Lemniscate, Viviani's CurveExplore with Wolfram|Alpha
Cite this as:
Weisstein, Eric W. "Figure Eight." From MathWorld--A Wolfram Resource. https://mathworld.wolfram.com/FigureEight.html