数学上，集合X上的对称群记作S_X或Sym(X)。它的元素是所有X到X自身的双射组成的群。由于恒等函数是双射，双射的反函数也是双射，并且两个双射的复合仍是双射，这个集合关于函数的复合成为群，即是置换群Sym(X)。两个函数的复合一般记作f o g，在置换群的表示里简记作fg。

对称群在很多不同的数学领域中，都扮演了重要角色。包括：伽罗华理论、不变量理论、李群的表示理论和组合学等等。

In abstract algebra, the symmetric group S_n on a finite set of n symbols is the group whose elements are all the permutation operations that can be performed on n distinct symbols, and whose group operation is the composition of such permutation operations, which are defined as bijective functions from the set of symbols to itself. Since there are n! (n factorial) possible permutation operations that can be performed on a tuple composed of n symbols, it follows that the order (the number of elements) of the symmetric group S_n is n!.