pe·ri·od / ˈpi(ə)rēəd/ • n. 1. a length or portion of time: he had long periods of depression the ale will be available for a limited period the period 1977–85. ∎ a portion of time in the life of a person, nation, or civilization characterized by the same prevalent features or conditions: the early medieval period. ∎ one of the set divisions of the day in a school allocated to a lesson or other activity. ∎ a set period of time during which a particular activity takes place: the training period is between 16 and 18 months. ∎ each of the intervals into which the playing time of a sporting event is divided. ∎ a major division of geological time that is a subdivision of an era and is itself subdivided into epochs, corresponding to a system in chronostratigraphy.2. a punctuation mark (.) used at the end of a sentence or an abbreviation. ∎ inf. added to the end of a statement to indicate that no further discussion is possible or desirable: he is the sole owner of the trademark, period.3. Physics the interval of time between successive occurrences of the same state in an oscillatory or cyclic phenomenon, such as a mechanical vibration, an alternating current, a variable star, or an electromagnetic wave. ∎ Astron. the time taken by a celestial object to rotate around its axis, or to make one circuit of its orbit. ∎ Math. the interval between successive equal values of a periodic function.4. (also menstrual period) a flow of blood and other material from the lining of the uterus, lasting for several days and occurring in sexually mature women (who are not pregnant) at intervals of about one lunar month until the onset of menopause.5. Chem. a set of elements occupying an entire horizontal row in the periodic table.6. Rhetoric a complex sentence, esp. one consisting of several clauses, constructed as part of a formal speech or oration. ∎ Mus. a complete idea, typically consisting of two or four phrases.• adj. belonging to or characteristic of a past historical time, esp. in style or design: a splendid selection of period furniture.ORIGIN: late Middle English (denoting the time during which something, esp. a disease, runs its course): from Old French periode, via Latin from Greek periodos ‘orbit, recurrence, course,’ from peri- ‘around’ + hodos ‘way, course.’ The sense ‘portion of time’ dates from the early 17th cent.
1. Second-order geologic time unit which is the equivalent of the chronostratigraphic unit ‘system’. Periods are subdivided into epochs; together, several periods constitute an era. When used formally the initial letter of the term is capitalized, e.g. the Devonian Period.
2. (T) The time that elapses between repetitions of the same phase of a wave-form, i.e. the time taken to complete one cycle. For a simple harmonic function, T = 2π/ω where ω is the angular velocity; for a wavetrain of single frequency f, T = 1/f = λ/V where λ is the wavelength and V the phase velocity.
So periodic(al) XVII. — F. or L. — Gr. periodikós. periodicity XIX. — F.