Closed form
A form \omega with d\omega=0.
A differential form \omega is closed if d\omega=0. Every exact form is automatically closed (since d(d\alpha)=0), but the converse can fail on a region that isn't simply connected -- exactly the gap de Rham cohomology measures.
Examples
The 1-form \dfrac{-y\,dx+x\,dy}{x^2+y^2} on the punctured plane is closed but not exact.
See also
Point table
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