Source : Webster's Revised Unabridged Dictionary (1913)

Plain \Plain\, v. i. [OE. playne, pleyne, fr. F. plaindre. See
   {Plaint}.]
   To lament; to bewail; to complain. [Archaic & Poetic]
   --Milton.

         We with piteous heart unto you pleyne.   --Chaucer.

Plain \Plain\, v. t.
   To lament; to mourn over; as, to plain a loss. [Archaic &
   Poetic] --Sir J. Harrington.

Plain \Plain\, a. [Compar. {Plainer}; superl. {Plainest}.] [F.,
   level, flat, fr. L. planus, perhaps akin to E. floor. Cf.
   {Llano}, {Piano}, {Plan}, {Plane} level, a level surface.]
   1. Without elevations or depressions; flat; level; smooth;
      even. See {Plane}.

            The crooked shall be made straight, and the rough
            places plain.                         --Isa. xl. 4.

   2. Open; clear; unencumbered; equal; fair.

            Our troops beat an army in plain fight. --Felton.

   3. Not intricate or difficult; evident; manifest; obvious;
      clear; unmistakable. ``'T is a plain case.'' --Shak.

   4.
      (a) Void of extraneous beauty or ornament; without
          conspicious embellishment; not rich; simple.
      (b) Not highly cultivated; unsophisticated; free from show
          or pretension; simple; natural; homely; common.
          ``Plain yet pious Christians.'' --Hammond. ``The plain
          people.'' --A. Lincoln.
      (c) Free from affectation or disguise; candid; sincere;
          artless; honest; frank. ``An honest mind, and plain.''
          --Shak.
      (d) Not luxurious; not highly seasoned; simple; as, plain
          food.
      (e) Without beauty; not handsome; homely; as, a plain
          woman.
      (f) Not variegated, dyed, or figured; as, plain muslin.
      (g) Not much varied by modulations; as, a plain tune.

   {Plain battle}, open battle; pitched battle. [Obs.]
      --Chaucer.

   {Plain chant} (Mus.) Same as {Plain song}, below.

   {Plain chart} (Naut.), a chart laid down on Mercator's
      projection.

   {Plain dealer}.
      (a) One who practices plain dealing.
      (b) A simpleton. [Obs.] --Shak.

   {Plain dealing}. See under {Dealing}.

   {Plain molding} (Join.), molding of which the surfaces are
      plain figures.

   {Plain sewing}, sewing of seams by simple and common
      stitches, in distinct from fancy work, embroidery, etc.;
      -- distinguished also from designing and fitting garments.
      

   {Plain song}.
      (a) The Gregorian chant, or {canto fermo}; the prescribed
          melody of the Roman Catholic service, sung in unison,
          in tones of equal length, and rarely extending beyond
          the compass of an octave.
      (b) A simple melody.

   {Plain speaking}, plainness or bluntness of speech.

   Syn: Level; flat; smooth; open; artless; unaffected;
        undisguised; frank; sincere; honest; candid; ingenuous;
        unembellished; downright; blunt; clear; simple;
        distinct; manifest; obvious; apparent. See {Manifest}.

Plain \Plain\, n. [Cf. OF. plaigne, F. plaine. See {Plain}, a.]
   1. Level land; usually, an open field or a broad stretch of
      land with an even surface, or a surface little varied by
      inequalities; as, the plain of Jordan; the American
      plains, or prairies.

            Descending fro the mountain into playn. --Chaucer.

            Him the Ammonite Worshiped in Rabba and her watery
            plain.                                --Milton.

   2. A field of battle. [Obs.] --Arbuthnot.

            Lead forth my soldiers to the plain.  --Shak.

Plain \Plain\, v. t. [imp. & p. p. {Plained}; p. pr. & vb. n.
   {Plaining}.] [Cf. {Plane}, v.]
   1. To plane or level; to make plain or even on the surface.
      [R.]

            We would rake Europe rather, plain the East.
                                                  --Wither.

   2. To make plain or manifest; to explain.

            What's dumb in show, I'll plain in speech. --Shak.

Plain \Plain\, adv.
   In a plain manner; plainly. ``To speak short and pleyn.''
   --Chaucer. ``To tell you plain.'' --Shak.

Source : WordNet®

plain
     adj 1: clearly apparent or obvious to the mind or senses; "the
            effects of the drought are apparent to anyone who sees
            the parched fields"; "evident hostility"; "manifest
            disapproval"; "patent advantages"; "made his meaning
            plain"; "it is plain that he is no reactionary"; "in
            plain view" [syn: {apparent}, {evident}, {manifest}, {patent}]
     2: not elaborate or elaborated; simple; "plain food"; "stuck to
        the plain facts"; "a plain blue suit"; "a plain
        rectangular brick building" [ant: {fancy}]
     3: lacking patterns especially in color [syn: {unpatterned}]
        [ant: {patterned}]
     4: not mixed with extraneous elements; "plain water"; "sheer
        wine"; "not an unmixed blessing" [syn: {sheer}, {unmingled},
         {unmixed}]
     5: free from any effort to soften to disguise; "the plain and
        unvarnished truth"; "the unvarnished candor of old people
        and children" [syn: {unvarnished}]
     6: lacking embellishment or ornamentation; "a plain hair
        style"; "unembellished white walls"; "functional
        architecture featuring stark unornamented concrete" [syn:
        {unembellished}, {unornamented}]
     7: lacking stylistic embellishment; "a literal description";
        "wrote good but plain prose"; "a plain unadorned account
        of the coronation"; "a forthright unembellished style"
        [syn: {literal}, {unembellished}]
     8: comprehensible to the general public; "written for the
        popular press in plain nontechnical language" [syn: {popular}]
     9: lacking in physical beauty or proportion; "a homely child";
        "several of the buildings were downright homely"; "a plain
        girl with a freckled face" [syn: {homely}]

plain
     adv : unmistakably (`plain' is often used informally for
           `plainly'); "the answer is obviously wrong"; "she was
           in bed and evidently in great pain"; "he was manifestly
           too important to leave off the guest list"; "it is all
           patently nonsense"; "she has apparently been living
           here for some time"; "I thought he owned the property,
           but apparently not"; "You are plainly wrong"; "he is
           plain stubborn" [syn: {obviously}, {evidently}, {manifestly},
            {patently}, {apparently}, {plainly}]

plain
     n 1: extensive tract of level open land; "they emerged from the
          woods onto a vast open plain"; "he longed for the fields
          of his youth" [syn: {field}, {champaign}]
     2: a basic knitting stitch [syn: {knit}, {knit stitch}, {plain
        stitch}]

plain
     v : express complaints, discontent, displeasure, or unhappiness;
         "My mother complains all day"; "She has a lot to kick
         about" [syn: {complain}, {kick}, {sound off}, {quetch}, {kvetch}]
         [ant: {cheer}]

Source : Free On-Line Dictionary of Computing

PLAIN
     
        Programming LAnguage for INteraction.  Pascal-like, with
        extensions for database, string handling, exceptions and
        pattern matching.  "Revised Report on the Programming Language
        PLAIN", A. Wasserman, SIGPLAN Notices 6(5):59-80 (May 1981).