What S means in S-duality? As I know, there are many dualities related to S-duality. For example, Montonen-Olive duality, Seiberg duality. and so on.
so, I wonder that what "S" means in the term "S-duality".
If this is a stupid question, I am really sorry.
 A: Quoting from Joseph Polchinski's book String Theory Vol 2.

S-duality : a duality under which the coupling constant of a quantum theory changes nontrivially, including the case of weak-strong duality. Important examples are the $SL(2,\mathbb{Z})$ self-dualities of IIB string theory and $d = 4, N = 4$ supersymmetric Yang-Mills theory. More loosely, it is used for weak-strong dualities between different theories,$\ldots$.

S-duality is a duality concerning the coupling constant (i.e. strength of interaction). The strong coupling limit of one theory is equivalent to the weak coupling limit of another  theory. The S here stands for the strong as appear in weak-strong duality.
The origin of this term probably stems from the dualities in the s-, t- and u-channel in scattering matrices. There are other type of dualities
which is named following this s/t/u pattern.


*

*T-duality - a duality concerning the length scale, usually in a toroidally compactified theory.
For an example, consider a closed string living on a space where one of its dimensions has been compactified to a torus of radius $R$. Its momentum along that dimension is quantized. Its mass contains a term $\propto \frac{n}{R}$ which $n$ is the level of excitation along the direction. 
The string can also wrap around that dimension with a winding number $w$. The tension of the string contribute another part $\propto wR$ to the mass.
In the limit of small and large geometry ( $R \to 0$ vs $R \to \infty$), these two pieces $\frac{n}{R}$ and $wR$ can exchange their role and relate the physics on different end.

*U-duality - a "unified" duality, a duality that is neither an S- nor a T-duality. 
For example, a transformation that exchanges a large geometry of one  theory with the strong coupling of another theory.
A: See this wikipedia article. It is probable the "S" stands for Spanier as in Edwin Spanier.
