\begin{definition}[Definition:Pairwise Balanced Design]
Let $\struct {X, \BB}$ be a design.
Let $\card X \ge 2$.
Then $\struct {X, \BB}$ is a '''pairwise balanced design''', abbreviated '''PBD''', {{iff}} every two distinct points in $X$ belong to exactly $\lambda$  blocks, for some constant $\lambda > 0$.
That is, {{iff}} any pair of distinct points appears in the same number of blocks as any other pair of distinct points.
Category:Definitions/Design Theory
\end{definition}