A period is an arbitrary interval of time. The word is applied to many different concepts:

• generally, in science, the time taken for one complete cycle of an alternating quantity. The period of oscillation of a wave is the time taken for the wave to complete one wavelength. Period is the reciprocal of the frequency. See amplitude, wavelength, simple harmonic motion.
• in astronomy, Copernicus used period to refer to the time it takes a planet to complete one orbit. See orbital period.
• in mathematics, the period of a function is the length of the interval over which it reappears. See periodic function.
• in chemistry, the term period is often used to mean a periodic table period, a row of the periodic table.
• in geology to identify named timespans such as the Cretaceous Period or the Neogene Period. Periods are generally longer than Epochs and shorter than Eras. The term Age is sometimes used more or less interchangeably with Period. See geologic period and geologic timescale to put this in perspective.
• the word period is often used in to refer to discrete portions of human history, which are also often called ages or eras (see also era). Historical periods include the following: Prehistory, Stone age, Bronze Age, Ice age, Iron Age, Ancient history, Middle Ages, Dark age, Golden age, Edwardian period, Elizabethan era, Victorian era, Information Age, Little Ice Age, Viking Age, The Age of Reason.
• in education, a short period of teaching in a particular subject. See lesson.
• Period is also used in a more vague fashion by interior decorators and designers in the form of "period" decorating, or "period" furniture, which may be from any historical period, or even "old-fashioned".

Further meanings less connected with time:

• in rhetoric, refers to well-balanced sentences. See period (rhetoric).
• in writing, a period was formerly a name for a long sentence, and from that it is now another name for the full stop punctuation sign: "." because it marks the end of a sentence.
• also in writing, a period can refer to any dot.
• a colloquial term for menstruation.
• in music a period is in some ways equivalent to a period in writing: it closes a phrase of music, though the "period" may also be the cadence or close of the phrases.