A (usually liquid) rocket propellant that can be used by itself, without the need for a second component. Monopropellants can either be a single chemical that can be made to decompose exothermically, or a mixture of chemicals (generally a fuel and an oxidizer) that can be made to react with one another and release energy. The most common monopropellants currently used are hydrogen peroxide and hydrazine, both of which are generally decomposed with a catalyst bed or thermal bed in a rocket engine to produce thrust.
Much work was done in the US in the 1950s and 1960s to attempt to find better and higher-energy monopropellants. For the most part, the people working on monopropellants came to the conclusion that any single substance that contained enough energy to compete with bipropellants would be too unstable to handle safely under practical conditions.
There is an entire chapter on the history of monopropellant development in the book "Ignition! An Informal History of Liquid Rocket Propellants" by John D. Clark, first published in 1972, ISBN 0-8135-0725-1. It is unfortunately currently out of print, but can be ordered from UMI Books on Demand.
Also see: Monopropellant_rocket