Definition:Pairwise Balanced Design

Definition
Let $\struct {X, \BB}$ be a design.

Let $\card X \ge 2$.

Then $\struct {X, \BB}$ is a pairwise balanced design, abbreviated PBD, every two distinct points in $X$ belong to exactly $\lambda$  blocks, for some constant $\lambda > 0$.

That is, any pair of distinct points appears in the same number of blocks as any other pair of distinct points.