A line integral operator representing integration around a closed contour or boundary in complex analysis and vector calculus.
Examples
Line integral around a closed curve C
\oint_C \mathbf{F} \cdot d\mathbf{r} = \oint_C (P\,dx + Q\,dy)
Green's theorem in vector calculus
\oint_C (M\,dx + N\,dy) = \iint_D \left(\frac{\partial N}{\partial x} - \frac{\partial M}{\partial y}\right)\,dx\,dy
Complex contour integral
\oint_{|z|=1} \frac{dz}{z} = 2\pi i