Unconditioned stimulus is generally defined as a natural response to stimuli without prior conditioning or any alteration to instinctive responses. The principal of unconditioned stimulus it is important in the areas of behaviorism, psychoanalysis, and measuring the effect of condition stimuli response.
Perhaps the best known example of a conditioned stimulus is Pavlov's dog. The dog was not naturally inclined to salivate at the sound of the bell, my conditioning reduced that response after training. Unconditioned stimulus, by contrast is quite literally the exact opposite of this form of conditioning and may be used as a control in researching response to stimuli.