conditioned stimulus. Definition: in classical conditioning, an originally irrelevant stimulus that, after assocation with a US, comes to trigger a conditioned response Researcher: Ivan Pavlov Example: the previously neutral tone stimulus that now triggered the conditional salivation.  .efinition: in classical conditioning, a stimulus that unconditionally (naturally and automatically) triggers a response Researcher: Ivan Pavlov Example: food in mouth automatically triggers the salivary reflex.