A monopropellant is a chemical (or a mixture of chemicals) used for rocketry or other propulsion systems to create thrust. They are ignited by heating or passing through a catalyst.
These sorts of chemicals tend to be unstable, and hence somewhat dangerous, although if handled correctly are often quite safe.
Common examples are hydrogen peroxide or Hydrazine.
Monopropellants generally have about half the performance of bipropellants (which require 2 tanks and more plumbing), but the systems are often simpler, cheaper, and perhaps lighter and more reliable. Also, most monopropellants are storable in space, whereas propellents like LOX or liquid hydrogen tend to boil away.
An unusual example of a
monopropellant is aluminum powder floating in
LOX. As with all metal powders aluminum burns very well, in LOX even more so, but the mixture is much more stable than you might expect, even if you throw gravel in with it (gravel tends to take the oxide layer off the aluminum, and make it more reactive).