These propellants combust on contact with each other. They do not require an ignition source. This property simplifies the design of the rocket motors that use hypergolic propellants but makes storing and handling them hazardous. Examples include Unsymmetrical Dimethylhydrazine (UDMH) and Dinitrogen Tetroxide (N2O4).
Pichon, Servane, Laurent Catoire, Nabiha Chaumeix, and Claude Paillard. "Search for green hypergolic propellants: gas-phase ethanol/nitrogen tetroxide reactivity." Journal of propulsion and power 21, no. 6 (2005): 1057-1061.