a portion of time

a chronological division; a stage of history

(used before a noun) of a particular historical period

period costume

a division of geological time that is part of an era and is subdivided into epochs

any of the divisions of the school day

a single occurrence of menstruation

chiefly NAmer a full stop

in physics, the interval of time that elapses before a cyclic motion or phenomenon begins to repeat itself

in mathematics, a number that does not change the value of a periodic function when added to the independent variable; esp the smallest such number

in chemistry, a horizontal row of elements in the periodic table

in astronomy, the time taken by a body, e.g. a planet, to make one complete rotation on its axis

in rhetoric, a well-proportioned sentence of several clauses