Pavlov noticed that when he put food in the dog's mouth, the dog salivated. and later the dog started to salivate even at the presence of the food bowl, etc.
Unconditioned Stimulus
The stimulus that naturally evokes a response in the subject.
Neutral Stimulus
The Irrelevant Stimulus that accompanies the UCS so as to create an association between the two.
Unconditioned Response
The natural response to the UCS
Conditioned Stimulus
Originally irrelevant Stimulus that was associated with the UCS, that now evokes a conditioned response