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 often 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.