Absolutely closed nil-2 groups
  Using the description of dominions in the variety of nilpotent groups of class at most two, we give a characterization of which groups are absolutely closed in this variety. We use the general result to derive an easier characterization for some subclasses; e.g. an abelian group $G$ is absolutely closed in ${\cal N}_2$ if and only if $G/pG$ is cyclic for every prime $p$. 
Arturo Magidin