The term "resilience" has its origin in metallurgy:  meaning, in metallurgical technology, the capacity of a metal to resist deformation from external forces.  For a metal, resilience represents the opposite of fragility.  This is valid also in the psychological field:  a resilient person is the opposite of one easily vulnerable.  Etymologically, "resilience" comes from Latin "resalio", iterative of "salio".  Some propose a suggestive connection between the original meaning of "salio", which also connotes the gesture of climbing aboard an upturned ship after capsizing from the force of the sea, and the actual use in the psychological field: both terms indicate a behavior of forging ahead in the face of adversity, without giving-up.

My own personal definition of the term is the following: 

Psychological resilience is the ability to persist in pursuing challenging objectives, affronting in an efficient manner the difficulties and other negative events which one encounters along the way.  The verb "persist" indicates the idea of a solid, unwavering motivation.  In fact, the resilient individual presents a series of unmistakable psychological characteristics: he/she is optimistic and tends to "read" negative events as temporary and limited; he/she believes to possess a wide margin of control on his/her own life as well as the surrounding environment; he/she is strongly motivated to achieve pre-established goals; he/she views change as a challenge and as an opportunity instead as a threat; when faced with defeat and frustrations he/she is capable of maintaining hope.