The term measurement has been used in a number of different sub-areas of psychology without an explicit recognition of the commonalities and potential differences in measurement characteristics. Analysis of these measurement situations reveals that the one common factor is that a mechanism or discriminative process, that is a standard system of measurement, is invariably used to identify magnitudes. A standard system is any mechanism or discriminative process that can indicate magnitude or differences between magnitudes. Three types of standard systems can be identified in psychology: the human, or elementary; the devised; and a dual process type. In addition, measurement situations may be differentiated in terms of different identifying characteristics or through differences in the way the standard system denotes the attribute being measured. Measurement may be identified in seven categories. Explicit identification of these categories of measurement in psychology and the elements of the measurement process allows for systematic comparison between the categories. They may be shown to differ to some extent in terms of denotability of the characteristic under consideration, the operating features of the standard system, the way in which metric or numerical information is derived, and the type of procedures used to evaluate measurement accuracy. (Author/PN)