UR, or Unconditioned Reflex - The response to a specific stimulus (the unconditioned stimulus) that the organism does not need to learn.

In other words, when a stimulus causes a natural, unlearned reaction, that reaction is called an unconditioned response. Unconditioned responses, are inherent behaviors or behaviors that occur naturally, from birth, in response to a given unconditioned stimulus.

An example of an unconditioned response is that of Pavlov's dog salivating when it sees or smells food. The dog will do this from the very first time it sees or smells food after its birth. The dog does not have to learn this behavior. That is what makes it an unconditioned response.