Nomenclature for simple types of learning is ambiguous and incomplete, as even commonly used terms such as 'habituation' and 'sensitization' are not applied consistently. One problem is a failure to distinguish between operational and theoretical constructs linguistically. Operational terminology for reporting behavioural results should be different from the language used in the discussion of learning theory. Thus, systematic operational terminology for simple types of conditioning is proposed. The most general category, stimulus exposure (SE) conditioning, is learning by exposure to a stimulus or to multiple stimuli where explicit inter-stimulus contingencies or instrumental reinforcers are not a part of training. Subtypes of SE conditioning are distinguished by the number of different stimuli used during training, by the method used to assess learning, and by the relationship between training stimuli and the assessment method. These categories include 'alteractive', 'iterative', 'heterostimic', and 'multistimic' conditioning. Learned responses are also categorized as reduction, enhancement, or transformation. SE conditioning categories combine with response terminology in phrases such as 'iterative reduction', which is a decrease in the response to a stimulus due to repeated presentation of that stimulus. (C) 1998 Elsevier Science B.V. All rights reserved.