Monostable multivibrators are multivibrators with only a single stable state. This state can either be high or low depending on the implementation of the capacitive, resistive, and transistor network.  However, the circuit is a two state circuit, and can be in an opposite state as well.  This state is the unstable state, which in order to be entered must be triggered by a signal from outside the multivibrator circuitry. 

The advantage of a multivibrator of this type is that the amount of time that it enters an unstable state is consistent according to the RC constant of the circuit.  In other words, when the unstable state is triggered by the appropriate signal, a pulse is outputted for a specific time before the device returns to the stable state.

Monostable Multivibrators can be considered a type of oscillation circuit even though it internally does not produce an oscillation pattern.  Most are used with a trigger pulse, which times precisely the initiation of the multivibrator pulse.  The multivibrator output frequency will, therefore, match the trigger frequency; with the only requirement being the wavelength of the trigger must be larger than the pulse width of the multivibrator.