A common point of two edges in a graph drawing that is not an incident vertex is called a crossing.
The crossing number $cr(G)$ is defined to be the minimum number of crossings in any drawing of $G$.
The pairwise crossing number of $G$, denoted with $pcr(G)$, is the minimum number of pairs of edges $(e_1, e_2) \in E \times E$, $e_1 \neq e_2$ such that $e_1$ and $e_2$ determine at least one crossing, over all drawings of $G$.
Can you give an example to show pairwise crossing number is not equal to crossing number? i.e. $pcr(G)\neq cr(G).$