What is Environmental Robustness? Is it different from plasticity?
I think robustness and plasticity are different concepts, although related to each other.

I would define plasticity as the property of a system to adapt to external changes. As defined in the wikipedia page for phenotypic plasticity:


  Phenotypic plasticity is the ability of an organism to change its phenotype in response to changes in the environment.


I think this definition agrees with the one in your question.

Robustness in biological systems, however, is defined as (wikipedia article):


  Robustness of a biological system (also called biological or genetic robustness) is the persistence of a certain characteristic or trait in a system under perturbations or conditions of uncertainty.


From this article three references are cited, all of which give some definition of robustness in their introductions/abstracts:

Felix and Wagner (Heredity 2008):


  Robustness, the persistence of an organismal trait under perturbations, is a ubiquitous property of complex living systems.


Kitano (Nature Reviews Genetics 2004):


  Robustness is a property that allows a system to maintain its functions despite external and internal perturbations.


Stelling et al. (Cell 2004):


  Robustness, the ability to maintain performance in the face of perturbations and uncertainty, is a long-recognized key property of living systems.


All these definitions imply that robustness is the ability to remain unperturbed under external or environmental perturbations. In a sense this looks like the opposite to plasticity, which is the ability to change under external perturbations. Personally, I think that robustness is accomplished because biological plasticity allows organisms to compensate (with some limits) the external perturbation in order to maintain their state.