The connection established by a subject or learner between an action and a result. Thorndike considered the stimulus-response association and trial-and-error to be the two basic types of intelligence.
Thorndike investigated the learning curves of cats escaping from his puzzle boxes. The cat learned to escape from the puzzle box by pressing a lever, associating movements produced through trial and error with the successful outcome-escape. Each success strengthened satisfying behaviour and weakened annoying behaviour, a finding summarised in his law of effect.
No cat can form an association leading to an act unless there is included in the association an impulse of its own which leads to the act.
(Thorndike, 1911, Chapter 2)