The phrase "unconditioned stimulus" refers to something that naturally triggers a response in a person or animal without any prior learning or conditioning. It is a stimulus that brings about an automatic reaction. Full definition
Thus, tail shock (the unconditioned stimulus) produces greater facilitation of the monosynaptic EPSP from a siphon sensory neuron to a siphon motor neuron if the shock is preceded by spike activity in the sensory neuron than if the shock and spike activity occur in a specifically unpaired pattern or if the shock occurs alone. (science.sciencemag.org)
This reflex exhibits classical conditioning when a weak tactile stimulus to the siphon is used as a conditioned stimulus and a strong shock to the tail is used as an unconditioned stimulus. (science.sciencemag.org)
Rony Paz and colleagues studied non-human primates trained to associate an aversive odour (the unconditioned stimulus) with a tone (the conditioned stimulus). (natureasia.com)