Order (Difference Set)

Let $G$ be Group of Order $h$ and $D$ be a set of $k$ elements of $G$. If the set of differences $d_i-d_j$ contains every Nonzero element of $G$ exactly $\lambda$ times, then $D$ is a $(h,k,\lambda)$-difference set in $G$ of order $n=k-\lambda$.