Monostable: Single stable state
Bistable: Two stable states
Astable: No stable state means oscillates automatically
[ Single stable state, if output distracted from its stable state, it will again come back to its stable state.]
It has only one stable state – either high or low. Let us consider that, we have designed a monostable multivibrator taking low as stable. Now if we make the output high by giving an input pulse, then it again returns to its stable state (i.e. low).
How long it will remain high can be calculated by the simple formulae of capacitor charging.