Of course "flatness" is a word that evokes a very particular geometric picture, and it seems to me like there should be a reason why this word is used, but nothing I can find gives me a reason!


Is there some geometric property corresponding to "flatness" (of morphisms, modules, whatever) that makes the choice of terminology obvious or at least justifiable?



Why are flat morphisms "flat?"
A lot of people will tell you that flatness means "continuously varying fibres" in some sense, and that flatness was invented to have correspondingly nice *consequences*, which is true. But there is a way to expect this (vague) interpretation *a priori* from an alternative, equivalent definition:


An $A$-module $M$ is flat $\iff$ $I \otimes\_A M \to IM$ is an isomorphism for every ideal $I$.


I would prefer to present this as the definition of flatness, and present the fact that tensoring with $M$ preserves exact sequences as a theorem. Why?


Thinking "geometrically", $I$ just corresponds (uniquely) to a closed subscheme $Z=Z(I)=$   
$=Spec(A/I)\subseteq Spec(A)$. If we think of $M$ in the usual geometric way as a module of generalized functions on $X$ (like sections of a bundle), and $M/IM \simeq M\otimes\_A A/I$ as its restriction to $Z$, then the above definition of flatness can be interepreted directly to mean that  **$M$ restricts nicely to closed subschemes $Z$**.


More precisely, it says that what we lose in this restriction, the submodule $IM$ of elements which "vanish on $Z$", is easy to understand: it's just formal linear combinations of elements $i\otimes m$, with no surprise relations among them, i.e. the tensor product $I \otimes\_A M$.


In topology, continuous functions "restrict nicely" to points and closed sets (by taking limits), so you can see, without much experience at all, how this definition corresponds in an intuitive way to continuity.


Having this motivation in place, the best thing to do is to check out examples along the lines of Dan Erman's answer to see the analogy with continuity and limits at work.


