# What is the difference between a variety and a manifold?

I hear people use these words relatively interchangeably. I’d believe that any differentiable manifold can also be made into a variety (which data, if I understand correctly, implicitly includes an ambient space?), but it’s unclear to me whether the only non-varietable manifolds should be those that don’t admit smooth structures. I’d hope there’s more to it than that.

I’ve heard too that affine schemes are to schemes local coordinates are to manifolds, so maybe my question should be about schemes instead — I don’t even know enough to know…

In English (as opposed to French, in which language variety and manifold are synonyms) the word variety is short for algebraic variety. The main differences, then, between (algebraic) varieties and (smooth) manifolds are that:

(i) Varieties are cut out in their ambient (affine or projective) space as the zero loci of polynomial functions, rather than simply as the zero loci of smooth functions. This gives them a more rigid structure. (Here I am thinking just of quasi-projective varieties; there are objects that people call varieties which can’t be immersed into projective space, but there is no need to think about them when you are just learning the subject. Also, a manifold need not be regarded as lying in an ambient Eulcidean space, but can always be immersed into one, and can then be thought of as being cut out as the zero locus of smooth
functions.)

The rigidity of varieties is reflected in the definition of isomorphism: we define two varieties to be isomorphic if we can find polynomial maps giving rise to mutually inverse bijections from one to the other, while two manifolds are isomorphic (i.e. diffeomorphic) if we can find smooth maps giving rise to mutually inverse bijections between them. It turns out, for example, that
the only diffeomorphism invariant of a compact connected surface is its genus $g$, while if we look at smooth connected projective curves over the complex numbers (which, when we forget the variety structure and just think of them as manifolds, are
compact connected surfaces — note that one complex dimension gives two real
dimensions) then the genus is not a complete invariant. For a fixed genus $g \geq 2$,
there is a $6g-6$-dimensional family of non-isomorphic curves of genus $g$.
(When $g = 1$ there is a $2$-dimensional family, and when $g = 0$, the
variety structure is actually uniquely determined. Also, by “dimension” here
I mean real dimension; but these families have their own natural algebraic variety structures, of half the dimension — i.e. there is a $3 g – 3$ dimensional variety parameterizing isomorphism classes of genus $g$ curves
when $g \geq 2$. Again the halving of dimension reflects the difference between real and complex dimension.)

(ii) Varieties can admit singularities, whereas we stipulate that manifolds be non-singular (i.e. locally Euclidean). Here it is useful to think about the fact that the critical locus of a (collection of) smooth function(s) can be pretty nasty, and so if we consider the zero loci of smooth functions and allow singularities, we will allow extremely nasty singularities. On the other hand,
the critical locus of a (collection) of polynomial(s) is not so bad (e.g. it is always of codimension at least one in the zero locus), and so allowing singularities in the theory turns out to be okay (and in fact to be more than okay; it turns out to be one of the more powerful features of algebraic geometry).