τ (tau) is a geometric constant approximately equal to 6.283. The number is naturally occuring as any circle’s circumference divided by its radius.
In radians, a unit which measures angles, one full rotation is equal to τ radians. This is demonstrated by the polar coordinate system interactive below which represents the position of a point as a distance form the origin (radius) and an angle in radians. Note, for the current given radius the arc-length along the circumference of the circle is equal to the current angle in radians multiplied by the radius.