Definition:Set Partition/Definition 1

Definition
Let $S$ be a set.

A partition of $S$ is a set of subsets $\Bbb S$ of $S$ such that:


 * $(1): \quad$ $\Bbb S$ is pairwise disjoint: $\forall S_1, S_2 \in \Bbb S: S_1 \cap S_2 = \O$ when $S_1 \neq S_2$
 * $(2): \quad$ The union of $\Bbb S$ forms the whole set $S$: $\displaystyle \bigcup \Bbb S = S$
 * $(3): \quad$ None of the elements of $\Bbb S$ is empty: $\forall T \in \Bbb S: T \ne \O$.

Also defined as
Some sources do not impose the condition that all sets in $\Bbb S$ are non-empty.

This is most probably more likely to be an accidental omission rather than a deliberate attempt to allow $\O$ to be an element of a partition.

The point is minor; proofs of partitionhood usually include a demostration that all elements of such a partition are indeed non-empty.

Also see

 * Equivalence of Definitions of Set Partition