Definition of Hypergolic fuel

A rocket propellant combination used in a rocket engine is called hypergolic when the propellants spontaneously ignite when they come into contact with each other. Strictly speaking it is the combination that is hypergolic, but in less precise usage the individual propellants are also referred to as hypergolic. The two propellant components usually consist of a fuel and an oxidizer. Although hypergolic propellants tend to be difficult to handle because of their extreme toxicity and/or corrosiveness, a hypergolic engine is relatively easy to ignite reliably.