adj. 1 appearing or occurring at regular intervals. 2 of or concerning the period of a celestial body (periodic motion). 3 (of diction etc.) expressed in periods (see PERIOD n. 7a). periodic decimal Math. a set of figures repeated in a recurring decimal. periodic function Math. a function returning to the same value at regular intervals. periodic table an arrangement of elements in order of increasing atomic number and in which elements of similar chemical properties appear at regular intervals. periodicity n.
[ F périodique or L periodicus f. Gk periodikos (as PERIOD) ]