In a preorder or poset$P$, an antichain is a subset $S \subseteq P$ such that no two distinct elements of $S$ are comparable.

Assuming $P$ has a bottom element $0$, a strong antichain is a subset $S \subseteq P$ such that for distinct $a, b \in S$, the only lower bound of $\{a, b\}$ is $0$. This definition may be extended to posets $P$without a bottom element, by declaring $A \subseteq P$ to be a strong antichain if $A$ is a strong antichain in $P^+$, the poset formed by freely adjoining a bottom element to $P$.