Capability of a system to fulfill its mission, in a timely manner, in the presence of attacks (external), failures (internal), or accidents (environment). (Ellison et al. 1999)
Ellison, R.J., D.A. Fisher, R.C. Linger, H.F. Lipson, T. Longstaff, N.R. Mead 2009. Survivable Network Systems: An Emerging Discipline, Carnegie-Mellon Software Engineering Institute Technical Report CMU/SEI-97-TR-013, 1997 revised 1999.