As I know the terms epigenetics is the study of gene expression changes without alteration of the DNA. Enviromental factor is an integral part in epigenetic theories. So they are linked, although not the same.
Enviromental IS the factor itself, epigenetic is a type of phenotypical change.
As well not all changes in gene expression are caused by enviromental factors (I don't think) and not all enviromental factors can be considered epigenetic.
Mind you, I'm not by any means an authority on epigenetics this is just what the terms mean to me. Maybe someone with more knowledge will come along sometime.