In differential geometry, the third axis in the Frenet-Serret frame of a curve, perpendicular to both the tangent and normal vectors.
From bi- 'two' + normal (from Latin normalis 'according to a rule'). Named because it's perpendicular to two other fundamental directions of a curve.
The binormal vector is the unsung hero of advanced calculus—it describes how a curve twists in 3D space, and it's essential for understanding everything from roller coaster design to how DNA spirals.
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