See here

The $p$th de Rham cohomology group of a manifold $M$ is the Quotient vector space:

$H^p(M) = \frac{\text{Ker} \: d: \Omega^p(M) \to \Omega^{p+1}(M)}{\text{Im}\:d: \Omega^{p-1}(M) \to \omega^p(M)}$