generally in differential cohesion
An étalé space (or étale map) over is an object in such that is a local homeomorphism: that is, for every , there is an open set such that the image is open in and the restriction of to is a homeomorphism .
The set where is called the stalk of over .
The underlying set of the total space (from ‘espace’) is the union of its stalks (notice that we do not say fiber!). is sometimes refered to as the projection.
Let be in . The (local) sections of over an open set are the continuous maps such that . It is an elementary but central fact that for an étale map , the images of local sections form a base for the topology of the total space . The topology of is then typically non-Hausdorff.
The set of sections of over is denoted by and may be shown to extend to a functor where is the category of presheaves over . The functor has a left adjoint , whose essential image is the full subcategory of étalé spaces over . The essential image of the functor is the category of sheaves over , and this adjunction restricts to an equivalence of categories between and (that is, it is an idempotent adjunction).
If is a sheaf, then one sometimes calls the total space of the étalé space the space of the sheaf , having in mind the adjoint equivalence above. (This is also called the sheaf space or the display space; compare also a display morphism of contexts.) The associated sheaf functor decomposes as , and may be considered as an endofunctor part of an idempotent monad in whose corresponding reflective subcategory is .
Every covering space (even in the more general sense not requiring any connectedness axiom) is étalé but not vice versa:
for a covering space the inverse image of some open subset in the base needs to be, by the definition, a disjoint union of homeomorphic open sets in ; however the ‘size’ of the open neighborhoods over various in the same stalk required in the definition of étalé space may differ, hence the intersection of their projections does not need to be an open set, if there are infinitely many points in the stalk.
even if the the stalks of the étalé space are finite, it need not be locally trivial. For instance the disjoint union of a collecton of open subsets of a topological space with the obvious projection is étale, but does not have a typical fiber: the fiber over a given point has cardinality the number of open sets that contain this particular point.
In French, the verb ‘étaler’ means, roughly, to spread out; ‘-er’ becomes ‘-é’ to make a past participle. So an ‘espace étalé’ is a space that has been spread out over . On the other hand, ‘étale’ is a (relatively obscure, distantly related) nautical adjective that can be translated as ‘calm’ or ‘slack’. So a ‘fonction étale’ is a slack function, one which is kind of a homeomorphism, but perhaps only locally.
To quote from the Wiktionnaire française:
‘étale’ qualifie la mer qui ne monte ni ne descend à la fin du flot ou du jusant (‘flot’ = ‘flow’ and ‘jusant’ = ‘ebb’).
There is an interesting stanza from a song of Léo Ferré:
Et que les globules figurent
Une mathématique bleue,
Sur cette mer jamais étale
D’où me remonte peu à peu
Cette mémoire des étoiles.
— (Léo Ferré, La mémoire et la mer)
He also mentions geometry and ‘théorème’ elsewhere in the song.