Before revision: The settling time of an amplifier or other output device (and more generally of a dynamical system in control theory) is the time elapsed from the application of an ideal instantaneous step input to the time at which the amplifier output has entered and remained within a specified error band.
After revision: In control theory the settling time of an amplifier or other output device a dynamical system such as is the time elapsed from the application of an ideal instantaneous step input to the time at which the amplifier output has entered and remained within a specified error band.