A path can bend through space yet feel perfectly straight to an ant walking the surface.
That turning inside the tangent plane is geodesic curvature k_g. When k_g = 0 the curve is a geodesic; the straightest possible route on S.
Its space curvature k then satisfies
k² = k_g² + k_n²
Robot motion, mesh processing and relativistic orbits all compute these zero-k_g curves.
Straightness is always measured against the geometry you inhabit.