  Let $X$ be a finite, simple graph with vertex set $V(X)$. The $2$-distance
graph $T_2(X)$ of $X$ is the graph with the same vertex set as $X$ and two
vertices are adjacent if and only if their distance in $X$ is exactly $2$. A
graph $G$ is a $2$-distance graph if there exists a graph $X$ such that
$T_2(X)=G$. In this paper, we give three characterizations of $2$-distance
graphs, and find all graphs $X$ such that $T_2(X)=kP_2$ or $K_m \cup K_n$,
where $k \ge 2$ is an integer, $P_2$ is the path of order $2$, and $K_m$ is the
complete graph of order $m \ge 1$.
