thumb|right|300px|The [[Bell number|15 partitions of a 4-element set ordered in a Hasse diagram There are S(4,1), ..., S(4, 4) = 1, 7, 6, 1 partitions containing 1, 2, 3, 4 sets.]]
In mathematics, particularly in combinatorics, a Stirling number of the second kind (or Stirling partition number) is the number of ways to partition a set of n objects into k non-empty subsets and is denoted by <math>S(n,k)</math> or <math>\textstyle \left\