The Collaborative International Dictionary of English v.0.48:

Period \Pe"ri*od\, n. [L. periodus, Gr. peri`odos a going round,
   a way round, a circumference, a period of time; peri` round,
   about + "odo`s a way: cf. F. p['e]riode.]
   1. A portion of time as limited and determined by some
      recurring or cyclic phenomenon, as by the completion of a
      revolution of one of the heavenly bodies; a division of
      time, as a series of years, months, or days, in which
      something is completed, and ready to recommence and go on
      in the same order; as, the period of the sun, or the
      earth, or a comet; the period of an electromagnetic wave
      is the time interval between maxima.
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   2. Hence: A stated and recurring interval of time; more
      generally, an interval of time specified or left
      indefinite; a certain series of years, months, days, or
      the like; a time; a cycle; an age; an epoch; as, the
      period of the Roman republic.
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            How by art to make plants more lasting than their
            ordinary period.                      --Bacon.
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   3. (Geol.) One of the great divisions of geological time; as,
      the Tertiary period; the Glacial period. See the Chart of
      Geology.
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   4. The termination or completion of a revolution, cycle,
      series of events, single event, or act; hence, a limit; a
      bound; an end; a conclusion. --Bacon.
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            So spake the archangel Michael; then paused,
            As at the world's great period.       --Milton.
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            Evils which shall never end till eternity hath a
            period.                               --Jer. Taylor.
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            This is the period of my ambition.    --Shak.
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   5. (Rhet.) A complete sentence, from one full stop to
      another; esp., a well-proportioned, harmonious sentence.
      "Devolved his rounded periods." --Tennyson.
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            Periods are beautiful when they are not too long.
                                                  --B. Johnson.
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   Note: The period, according to Heyse, is a compound sentence
         consisting of a protasis and apodosis; according to
         Becker, it is the appropriate form for the coordinate
         propositions related by antithesis or causality.
         --Gibbs.
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   6. (Print.) The punctuation point [.] that marks the end of a
      complete sentence, or of an abbreviated word.
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   7. (Math.) One of several similar sets of figures or terms
      usually marked by points or commas placed at regular
      intervals, as in numeration, in the extraction of roots,
      and in circulating decimals.
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   8. (Med.) The time of the exacerbation and remission of a
      disease, or of the paroxysm and intermission.
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   9. (Mus.) A complete musical sentence.
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   10. (Sports) One of the specified time intervals into which a
       game is divided; as, there are three periods in a hockey
       game.
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   11. (Education) One of the specified time intervals into
       which the academic day is divided; as, my calculus class
       is in the first period.
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   12. The time interval during which a woman is menstruating,
       or the event of a single menstruation; as, her period was
       late this month.
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   The period, the present or current time, as distinguished
      from all other times.
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   Syn: Time; date; epoch; era; age; duration; limit; bound;
        end; conclusion; determination.
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