In classical conditioning, acquisition is the phase when a neutral stimulus first becomes linked to an unconditioned stimulus. The dog hears the metronome and then receives food. After enough pairings the metronome alone triggers salivation. That gradual strengthening is acquisition.

The process is not instantaneous. Early trials produce little change. Then the conditioned response strengthens, often following a negatively accelerating curve. Timing matters: the neutral stimulus must usually precede the unconditioned stimulus by a short interval. Contiguity alone is not enough; the organism must also treat the first stimulus as a reliable predictor.

Acquisition can be measured by the strength or frequency of the conditioned response. Once the association is established, further pairings may produce only modest gains. Extinction begins when the conditioned stimulus is presented alone and the response weakens.

• Initial learning of the stimulus-response link
• Requires repeated pairing
• Sensitive to temporal contiguity and contingency
• Precedes the asymptotic level of responding

The same term appears in operant conditioning for the early strengthening of a behavior by reinforcement, though the mechanisms differ.