NOUN (1)

1. a mechanism capable of returning to a safe state in case there is a failure or malfunction;


ADJECTIVE (2)

1. guaranteed not to fail;
- Example: "a fail-safe recipe for cheese souffle"

2. eliminating danger by compensating automatically for a failure or malfunction;
- Example: "a fail-safe device in a nuclear weapon to deactivate it automatically in the event of accident"

WordNet (r) 3.0 (2006):

fail-safe
    adj 1: guaranteed not to fail; "a fail-safe recipe for cheese
           souffle"
    2: eliminating danger by compensating automatically for a
       failure or malfunction; "a fail-safe device in a nuclear
       weapon to deactivate it automatically in the event of
       accident"
    n 1: a mechanism capable of returning to a safe state in case
         there is a failure or malfunction