From The Collaborative International Dictionary of English v.0.48 :
Robustness \Ro*bust"ness\, n.
The quality or state of being robust.
[1913 Webster]
From WordNet (r) 3.0 (2006) :
robustness
n 1: the property of being strong and healthy in constitution
[syn: robustness, hardiness, lustiness, validity]
2: the characteristic of being strong enough to withstand
intellectual challenge; "the lack of robustness in the
findings may be due to the small size of the sample"