The following lines emphasise the important characteristics of dynamic systems that link input and output. The latter terminology is that usually used by the engineer but the physicist would tend to refer to them as cause and effect and a biologist as stimulus and response. The terms are used as alternatives in the above definition in order to stress that, from the systems viewpoint, there is no basic difference between the particular systems studied by the engineer, the physicist and the biologist. In the definition, the input and the output may consist of matter, or energy, or information. These alternative categories are listed in different order for the input and the output respectively, to emphasise that input and output need not belong to the same category. In the case of dynamic systems, input and output must be defined in terms of a given time reference.