General terms:
* S-curve: This is the most common synonym, referring to a curve that changes direction, forming an "S" shape.
* Sinuous: This word implies a smooth, flowing curve, often with a serpentine quality.
* Curvilinear: This refers to any shape consisting of curves.
* Concave-convex: This describes a curve that has both concave and convex sections.
* Undulating: This implies a wave-like, up-and-down curve.
* Bifurcated: This means split into two branches, which might have a curved shape.
More specific terms:
* Parabola: A specific type of curve with a specific mathematical equation.
* Hyperbola: Another specific type of curve with a specific mathematical equation.
* Ellipse: A closed curve where the sum of the distances from any point on the curve to two fixed points is constant.
* Spiral: A curve that winds around a central point.
Figurative language:
* Serpentine: This term implies a winding, snake-like shape.
* Meandering: This suggests a curve that wanders aimlessly.
To give you a more accurate synonym, please provide more context about the double curve you are describing.