In algebraic topology, an ${\displaystyle \mathbb {S} }$-object (also called a symmetric sequence) is a sequence ${\displaystyle \{X(n)\}}$ of objects such that each ${\displaystyle X(n)}$ comes with an action of the symmetric group ${\displaystyle \mathbb {S} _{n}}$.

The category of combinatorial species is equivalent to the category of finite ${\displaystyle \mathbb {S} }$-sets (roughly because the permutation category is equivalent to the category of finite sets and bijections.)

By ${\displaystyle \mathbb {S} }$-module, we mean an ${\displaystyle \mathbb {S} }$-object in the category ${\displaystyle {\mathsf {Vect}}}$ of finite-dimensional vector spaces over a field k of characteristic zero (the symmetric groups act from the right by convention). Then each ${\displaystyle \mathbb {S} }$-module determines a Schur functor on ${\displaystyle {\mathsf {Vect}}}$.

This definition of ${\displaystyle \mathbb {S} }$-module shares its name with the considerably better-known model for highly structured ring spectra due to Elmendorf, Kriz, Mandell and May.^{[clarification needed]}