The primary concern in the design of a high-frequency amplifier is to ensure that the active device does NOT oscillate at any frequency, especially at the frequency of operation.
A two-port active device is unconditionally stable if both its input and output impedances
have positive real part for ANY passive source and load terminations. In S-parameter
terminology, it is to say
It can be proved, will be shown in below, that the necessary and sufficient conditions for unconditionally stable operation are
and 
in most of the practical situation, these conditions
are natural for a transistor, otherwise the transistor could have been oscillating under the
measuring termination.
and 
.
Note that a unilateral device, 
, is always unconditionally stable.