Definition from Colliers World English Dictionary:
ecology: the ability of an ecosystem to return to its original state after being disturbed
physics: the amount of potential energy stored in an elastic material when deformed
from Merriam-Webster's Medical Dictionary:
1 : the capability of a strained body to recover its size and shape after deformation caused especially by compressive stress
2 : an ability to recover from or adjust easily to misfortune or change
Origin from Online Etymology Dictionary: 1620s, from L. resiliens , prp. of resilire "to rebound, recoil," from re- "back" + salire "to jump, leap"
[Retrieved January 25, 2011, from Dictionary.com website]