Definition:Pairwise Balanced Design

Let $(X,\mathcal B)$ be a Design.

Let $|X| \ge 2$.

Then $(X,\mathcal B)$ is a pairwise balanced design, abbreviated PBD, if every two distinct points in $X$ belong to exactly $\lambda$ blocks, for some constant $\lambda > 0$.

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