Cycles of the third kind

. This paper delineates three kinds or senses of the cycle concept as used in social science theory and research. Cycles of the weak or first kind merely refer to fluctuations or state changes. Cycles of the moderate or second kind refer to fluctuations with some constant periodicity in fact or in principle. Cycles of the strong or third kind have several analytical properties that require explanation. I argue that the term cycle should be reserved for cycles of the second or third kinds and that the terms fluctuation or alternation should be used for cycles of the first kind. The paper concludes with some comments on sources of resistance to the cycle concept.