ESP is usually said to have three varieties: 'telepathy,' in which the knowledge is of events in another person's mind, 'clairvoyance,' in which the knowledge is of physical objects or states of affairs; and 'precognition' (telepathic or clairvoyant), where the knowledge relates to happenings still in the future. The word 'knowledge' is, however, not entirely appropriate, for there may be telepathic or clairvoyant 'interaction,' in which a person's mental state or actions may be influenced by an external state of affairs, though he does not 'know' or 'cognize' it.

Another American term is 'psychokinesis' (PK), the direct influence of mental events on physical events external to the agent's body. 'Psi' (from the Greek letter) is 'a general term to identify personal factors or processes in nature which transcend accepted laws' [Gay74]. It is sometimes used to cover both ESP and PK.