Ivan Pavlov (1849 - 1936) is best known for his work describing the psychological phenomenon of Classical Conditioning. From his work studying digestion in dogs, he observed that the dogs would salivate at the mere sight of food.
Pavlov originally discovered the idea of conditioning by accident - he noticed that dogs began to salivate even before they were presented with food. He later tested the concept using what became his most famous experiment. In that experiment he conditioning dogs to salivate in response to the sound of a bell. He did this by ringing a bell as he presented food, at which the dogs would respond by salivating. After several trials of the bell and food presented together, Pavlov rang the bell alone without presenting food and the dogs gave the usual salivary response. Salivation in response to the ringing of the bell is known as a "conditioned response"
Pavlov's work lay the foundation for Behaviorism, which dominated the field of psychology from the 19th century until the first half of the 20th century. He also introduced many conditioning terms, including Conditioned Response, Unconditioned Response, Conditioned Stimulus, Unconditioned Stimulus, and more.