The Collaborative International Dictionary of English v.0.48:

Periodic \Pe`ri*od"ic\ (p[=e]`r[i^]*[o^]d"[i^]k), Periodical
\Pe`ri*od"ic*al\ (p[=e]`r[i^]*[o^]d"[i^]*kal), a. [L.
   periodicus, Gr. periodiko`s: cf. F. p['e]riodique.]
   1. Of or pertaining to a period or periods, or to division by
      periods.
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            The periodical times of all the satellites. --Sir J.
                                                  Herschel.
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   2. Performed in a period, or regular revolution; proceeding
      in a series of successive circuits; as, the periodical
      motion of the planets round the sun.
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   3. Happening, by revolution, at a stated time; returning
      regularly, after a certain period of time.
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            The periodic return of a plant's flowering.
                                                  --Henslow.
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            To influence opinion through the periodical press.
                                                  --Courthope.
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   4. Acting, happening, or appearing, at fixed or somewhat
      variable intervals; recurring; as, periodical epidemics
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   5. (Rhet.) Of or pertaining to a period; constituting a
      complete sentence.
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   Periodic comet (Astron.), a comet that moves about the sun
      in an elliptic orbit; a comet that has been seen at two of
      its approaches to the sun.

   Periodic function (Math.), a function whose values recur at
      fixed intervals as the variable uniformly increases. The
      trigonomertic functions, as sin(x), tan(x), etc., are
      periodic functions. Exponential functions are also
      periodic, having an imaginary period, and the elliptic
      functions have not only a real but an imaginary period,
      and are hence called doubly periodic.

   Periodic law (Chem.), the generalization that the
      properties of the chemical elements are periodic functions
      of their atomic weights. "In other words, if the elements
      are grouped in the order of their atomic weights, it will
      be found that nearly the same properties recur
      periodically throughout the entire series." The following
      tabular arrangement of the atomic weights shows the
      regular recurrence of groups (under I., II., III., IV.,
      etc.), each consisting of members of the same natural
      family. The gaps in the table indicate the probable
      existence of unknown elements.

   Periodic table, Periodic table of the elements (Chem.), A
      tabular arrangement of the chemical elements, illustrating
      the periodic law, described above.
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   Note: Note: A modern version of the periodic table can be
         found at: http://pearl1.lanl.gov/periodic/default.htm
         [PJC] TABLE OF THE PERIODIC LAW OF THE CHEMICAL
         ELEMENTS (The vertical columns contain the periodic
         groups) Series1[ 2[ 3[ 4[ 5[ 6[ 7[ 8[ 9[ 10[ 11[ 12[
         --------------------------------------------------------------
         |I. II. III. IV. V. VI. VII. VIII. | RH4 RH3 RH3 RH
         |R2O RO R3O3 RO2 R2O5 RO3 R2O7 RO4
         --------------------------------------------------------------
         H
         1
         Li
         7
         Na
         23
         K
         39
         (Cu)
         63
         Rb
         85.2
         (Ag)
         (108)
         Cs
         133
         (-)
         (-)
         (Au)
         (197)

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   Note: A similar relation had been enunciated in a crude way
         by Newlands; but the law in its effective form was
         developed and elaborated by Mendelejeff, whence it is
         sometimes called Mendelejeff's law. Important
         extensions of it were also made by L. Meyer. By this
         means Mendelejeff predicted with remarkable accuracy
         the hypothetical elements ekaboron, ekaluminium, and
         ekasilicon, afterwards discovered and named
         respectively scandium, gallium, and germanium.
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   Periodic star (Astron.), a variable star whose changes of
      brightness recur at fixed periods.

   Periodic time of a heavenly body (Astron.), the time of a
      complete revolution of the body about the sun, or of a
      satellite about its primary.
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