undulation, wave - (physics) a movement up and down or back and forth

alteration, change, modification - an event that occurs when something passes from one state or phase to another; "the change was intended to increase sales"; "this storm is certainly a change for the worse"; "the neighborhood had undergone few modifications since his last visit years ago"

vicissitude - a variation in circumstances or fortune at different times in your life or in the development of something; "the project was subject to the usual vicissitudes of exploratory research"

allomerism - (chemistry) variability in chemical composition without variation in crystalline form

deviation, difference, divergence, departure - a variation that deviates from the standard or norm; "the deviation from the mean"

permutation, replacement, substitution, switch, transposition - an event in which one thing is substituted for another; "the replacement of lost blood by a transfusion of donor blood"

business cycle, trade cycle - recurring fluctuations in economic activity consisting of recession and recovery and growth and decline

daily variation - fluctuations that occur between one day and the next

diurnal variation - fluctuations that occur during each day

tide - something that may increase or decrease (like the tides of the sea); "a rising tide of popular interest"

unregularity, irregularity - not characterized by a fixed principle or rate; at irregular intervals

scintillation - the twinkling of the stars caused when changes in the density of the earth's atmosphere produce uneven refraction of starlight