universal structure

Let $L$ be a first order language, and let $R$ be an elementary class of $L$-structures.Let $\kappa$ be a cardinal. $R_{\kappa }$ be the set of structures from $R$ with cardinality less than or equal to $\kappa$.

Let $M\in R_{\kappa }$.Suppose that for every $N\in R_{\kappa }$ there is an embedding of $N$ into $M$.Then we say $M$ is universal.