as flatlutenet has mentioned ,with refrence to the NYquist stability criterion shows weather the system is oscillatory or not ,this is basiclly determined by studying the open loop transfer function of the desired system when the O.l TF HAS NO poles in the right hand planes then the encirclements of 1+j0 in a cw direction determines the stability of the system ,for a certain system if you are checking if it's the potential to oscillate or not ,both the loop gain >1 and loop phase =2π n should be checked at the suspected frequency ,where any of them to be violated the system willnot oscillate .for example in the analysis of a fabry perot cavity the distance between the 2 reflectors was chosen such that a complete round trip phase = multiples of 2 pi. elseif , though the active medium supplies a round trip gain > 1 , the distance was adjusted for distructive interference( RTPS of pi) the circuit will not oscillate ,so both conditions must be satisfied simultaniously and i/p reflection >1 alone is not satisfactory.
h0op