The Bismuth Condition: A Differential Symmetry Criterion for Vanishing Line Integrals Description This work introduces the Bismuth condition, a global differential symmetry imposed on the Jacobian of a function or vector field. The condition provides a unified criterion for predicting vanishing line integrals over closed curves, extending classical results from vector calculus and complex analysis. In classical calculus, vanishing of line integrals is fully characterized only in the conservative case, where the integral vanishes on all closed curves. Outside this setting, no general tool exists to determine in advance when a closed integral will vanish without explicit computation. The Bismuth condition fills this gap by identifying geometric families of closed curves on which the integral vanishes, even for non-conservative fields. The same condition applies uniformly to real vector fields and to complex-valued functions. In particular, holomorphic functions form a strict subclass of functions satisfying the Bismuth condition, showing that the condition generalizes the differential structure underlying classical complex analyticity. Beyond holomorphicity, the condition allows bounded, non-holomorphic functions to exhibit the same vanishing-integral behavior on closed curves. Unlike potential-based or topological criteria, the Bismuth condition is purely differential and does not rely on the existence of a potential function, holomorphicity, or local PDE constraints. Its consequences are global and geometric, arising from symmetry in the Jacobian rather than from integrability assumptions. This framework provides a single tool that: recovers classical vanishing results for conservative fields, predicts vanishing integrals in non-conservative settings, applies equally to vector calculus and complex-valued functions, and explains vanishing integrals as a consequence of differential symmetry. The work suggests new perspectives on line integrals, symmetry, and integrability, with potential applications in multivariable calculus, complex analysis, and geometric aspects of differential equations.