A curve which can be turned continuously inside an equilateral triangle. There are an infinite number of delta curves, but the simplest are
the circle and lens-shaped 
-biangle. All the 
curves of height 
have the same perimeter 
. Also, at each position of a 
curve turning in an equilateral
triangle, the perpendiculars to the sides at the points of contact are concurrent
at the instantaneous center of rotation.
Delta Curve
See also
Equilateral Triangle, Lens, Reuleaux Polygon, Reuleaux Triangle, RotorExplore with Wolfram|Alpha
References
Honsberger, R. Mathematical Gems I. Washington, DC: Math. Assoc. Amer., pp. 56-59, 1973.Referenced on Wolfram|Alpha
Delta CurveCite this as:
Weisstein, Eric W. "Delta Curve." From MathWorld--A Wolfram Web Resource. https://mathworld.wolfram.com/DeltaCurve.html