definition unconditioned stimulus
In classical conditioning, an unconditioned stimulus (US or UCS) is any stimulus that can evoke a response without the organism going through any previous learning; the response to the US (the unconditioned response) occurs naturally.n classical conditioning, an unconditioned stimulus (US or UCS) is any stimulus that can evoke a response without the organism going through any previous learning; the response to the US (the unconditioned response) occurs naturally.