Source : Webster's Revised Unabridged Dictionary (1913)

Periodic \Pe`ri*od"ic\, Periodical \Pe`ri*od"ic*al\, a. [L.
   periodicus, Gr. ?: cf. F. p['e]riodique.]
   1. Of or pertaining to a period or periods, or to division by
      periods.

            The periodicaltimes of all the satellites. --Sir J.
                                                  Herschel.

   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.

   3. Happening, by revolution, at a stated time; returning
      regularly, after a certain period of time; acting,
      happening, or appearing, at fixed intervals; recurring;
      as, periodical epidemics.

            The periodic return of a plant's flowering.
                                                  --Henslow.

            To influence opinion through the periodical press.
                                                  --Courthope.

   4. (Rhet.) Of or pertaining to a period; constituting a
      complete sentence.

   {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 wieghts. ``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.

Periodical \Pe`ri*od"ic*al\, n.
   A magazine or other publication which appears at stated or
   regular intervals.

Source : WordNet®

periodical
     adj : happening or recurring at regular intervals [syn: {periodic}]
           [ant: {aperiodic}]

periodical
     n : a publication that appears at fixed intervals