  We introduce a variation of metric dimension, called the multiset dimension.
The representation multiset of a vertex $v$ with respect to $W$ (which is a
subset of the vertex set of a graph $G$), $r_m (v|W)$, is defined as a multiset
of distances between $v$ and the vertices in $W$. If $r_m (u |W) \neq r_m(v|W)$
for every pair of distinct vertices $u$ and $v$, then $W$ is called an
m-resolving set of $G$. If $G$ has an m-resolving set, then the cardinality of
a smallest m-resolving set is called the multiset dimension of $G$, denoted by
$md(G)$. If $G$ does not contain an m-resolving set, we write $md(G) = \infty$.
  In this paper we present basic results on the multiset dimension. We obtain
some (sharp) bounds for multiset dimension of arbitrary graphs in term of its
metric dimension, order, or diameter. We provide some necessary conditions for
a graph to have finite multiset dimension, with an example of an infinite
family of graphs where those necessary conditions are also sufficient. We also
show that the multiset dimension of any graph other than a path is at least $3$
and finally we provide two families of graphs having the multiset dimension
$3$.
