There are many definitions of resilience from simple deterministic views of resilience anchored in Newtonian mechanics to far more dynamic views of resilience from a systems perspective, including insights from quantum mechanics and the sciences of complexity.  One baseline perspective of resilience sees it in terms of the viability of socio-ecological systems as the foundation for sustainability.  For those that are ready to look beyond resilience as the ability to return to the "normal state" before a disaster, take a look at:

http://www.resalliance.org/