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https://en.wikipedia.org/wiki/%C3%89tale_cohomology
Etale cohomology

History
Etale cohomology was suggested by Grothendieck (1960), using some suggestions by J.-P. Serre, and was motivated by the attempt to construct a Weil cohomology theory in order to prove the Weil conjectures. The foundations were soon after worked out by Grothendieck together with Michael Artin, and published as Artin (Artin 1962) and SGA 4.
Grothendieck used etale cohomology to prove some of the Weil conjectures (Dwork had already managed to prove the rationality part of the conjectures in 1960 using p-adic methods), and the remaining conjecture, the analogue of the Riemann hypothesis was proved by Pierre Deligne (1974) using ?-adic cohomology.

Further contact with classical theory was found in the shape of the Grothendieck version of the Brauer group; this was applied in short order to diophantine geometry, by Yuri Manin.
The burden and success of the general theory was certainly both to integrate all this information, and to prove general results such as Poincare duality and the Lefschetz fixed point theorem in this context.

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