Hypergolic

Hypergolic propellant refers to rocket propellants that spontaneously ignite when they come into contact with each other. Normally we call them the fuel and the oxidiser. While hypergolics are obviously very dangerous and require special handling on the ground, they are extremely useful in space because no ignition system is needed, which in itself would be complex, expensive, and take up mass  budget. The typical example in space applications is hydrazine, used together with nitrogen tetroxide.