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. 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, they can typically be stored as liquids at room temperature and hypergolic engines are easy to ignite reliably and repeatedly.
In contemporary usage, the terms "hypergol" or "hypergolic
propellant" usually mean the most common such propellant combination, dinitrogen tetroxide plus hydrazine and/or its relatives monomethyl hydrazine and unsymmetrical dimethylhydrazine.