"Pain is a private percept that arises in a conscious brain, typically in response to a noxious provoking stimulus, but sometimes in the absence of a stimulus.

The relation of the percept to the stimulus is variable, and depends on the individual's prior expectations and beliefs, and on his/her cognitive and emotional state, not just on the nature of the stimulus itself. The nervous system may react to noxious stimuli with autonomic changes (e.g. in blood pressure), and even with adaptive behavioral responses, in the absence of a conscious pain percept.

Likewise, there are circumstances in which the presence of pain is ambiguous, such as when the individual is unable to report on his/her conscious percept, or with reference to animals. In these situations, the word "nociception" is used instead of the word "pain" to express that the nervous system has detected the noxious stimulus without necessarily implying that a pain percept was evoked."

M. Devor.