Source : Webster's Revised Unabridged Dictionary (1913)

Open \O"pen\, a. [AS. open; akin to D. open, OS. opan, G. offan,
   Icel. opinn, Sw. ["o]ppen, Dan. aaben, and perh. to E. up.
   Cf. {Up}, and {Ope}.]
   1. Free of access; not shut up; not closed; affording
      unobstructed ingress or egress; not impeding or preventing
      passage; not locked up or covered over; -- applied to
      passageways; as, an open door, window, road, etc.; also,
      to inclosed structures or objects; as, open houses, boxes,
      baskets, bottles, etc.; also, to means of communication or
      approach by water or land; as, an open harbor or
      roadstead.

            Through the gate, Wide open and unquarded, Satan
            passed.                               --Milton

   Note: Also, figuratively, used of the ways of communication
         of the mind, as by the senses; ready to hear, see,
         etc.; as, to keep one's eyes and ears open.

               His ears are open unto their cry.  --Ps. xxxiv.
                                                  15.

   2. Free to be used, enjoyed, visited, or the like; not
      private; public; unrestricted in use; as, an open library,
      museum, court, or other assembly; liable to the approach,
      trespass, or attack of any one; unprotected; exposed.

            If Demetrius . . . have a matter against any man,
            the law is open and there are deputies. --Acts xix.
                                                  33.

            The service that I truly did his life, Hath left me
            open to all injuries.                 --Shak.

   3. Free or cleared of obstruction to progress or to view;
      accessible; as, an open tract; the open sea.

   4. Not drawn together, closed, or contracted; extended;
      expanded; as, an open hand; open arms; an open flower; an
      open prospect.

            Each, with open arms, embraced her chosen knight.
                                                  --Dryden.

   5. Hence:
      (a) Without reserve or false pretense; sincere;
          characterized by sincerity; unfeigned; frank; also,
          generous; liberal; bounteous; -- applied to personal
          appearance, or character, and to the expression of
          thought and feeling, etc.

                With aspect open, shall erect his head. --Pope.

                The Moor is of a free and open nature. --Shak.

                The French are always open, familiar, and
                talkative.                        --Addison.
      (b) Not concealed or secret; not hidden or disguised;
          exposed to view or to knowledge; revealed; apparent;
          as, open schemes or plans; open shame or guilt.

                His thefts are too open.          --Shak.

                That I may find him, and with secret gaze Or
                open admiration him behold.       --Milton.

   6. Not of a quality to prevent communication, as by closing
      water ways, blocking roads, etc.; hence, not frosty or
      inclement; mild; -- used of the weather or the climate;
      as, an open season; an open winter. --Bacon.

   7. Not settled or adjusted; not decided or determined; not
      closed or withdrawn from consideration; as, an open
      account; an open question; to keep an offer or opportunity
      open.

   8. Free; disengaged; unappropriated; as, to keep a day open
      for any purpose; to be open for an engagement.

   9. (Phon.)
      (a) Uttered with a relatively wide opening of the
          articulating organs; -- said of vowels; as, the ["a]n
          f["a]r is open as compared with the [=a] in s[=a]y.
      (b) Uttered, as a consonant, with the oral passage simply
          narrowed without closure, as in uttering s.

   10. (Mus.)
       (a) Not closed or stopped with the finger; -- said of the
           string of an instrument, as of a violin, when it is
           allowed to vibrate throughout its whole length.
       (b) Produced by an open string; as, an open tone.

   {The open air}, the air out of doors.

   {Open chain}. (Chem.) See {Closed chain}, under {Chain}.

   {Open circuit} (Elec.), a conducting circuit which is
      incomplete, or interrupted at some point; -- opposed to an
      uninterrupted, or {closed circuit}.

   {Open communion}, communion in the Lord's supper not
      restricted to persons who have been baptized by immersion.
      Cf. {Close communion}, under {Close}, a.

   {Open diapason} (Mus.), a certain stop in an organ, in which
      the pipes or tubes are formed like the mouthpiece of a
      flageolet at the end where the wind enters, and are open
      at the other end.

   {Open flank} (Fort.), the part of the flank covered by the
      orillon.

   {Open-front furnace} (Metal.), a blast furnace having a
      forehearth.

   {Open harmony} (Mus.), harmony the tones of which are widely
      dispersed, or separated by wide intervals.

   {Open hawse} (Naut.), a hawse in which the cables are
      parallel or slightly divergent. Cf. {Foul hawse}, under
      {Hawse}.

   {Open hearth} (Metal.), the shallow hearth of a reverberatory
      furnace.

   {Open-hearth furnace}, a reverberatory furnace; esp., a kind
      of reverberatory furnace in which the fuel is gas, used in
      manufacturing steel.

   {Open-hearth process} (Steel Manuf.), a process by which
      melted cast iron is converted into steel by the addition
      of wrought iron, or iron ore and manganese, and by
      exposure to heat in an open-hearth furnace; -- also called
      the {Siemens-Martin process}, from the inventors.

   {Open-hearth steel}, steel made by an open-hearth process; --
      also called {Siemens-Martin steel}.

   {Open newel}. (Arch.) See {Hollow newel}, under {Hollow}.

   {Open pipe} (Mus.), a pipe open at the top. It has a pitch
      about an octave higher than a closed pipe of the same
      length.

   {Open-timber roof} (Arch.), a roof of which the
      constructional parts, together with the under side of the
      covering, or its lining, are treated ornamentally, and
      left to form the ceiling of an apartment below, as in a
      church, a public hall, and the like.

   {Open vowel} or {consonant}. See {Open}, a., 9.

   Note: Open is used in many compounds, most of which are
         self-explaining; as, open-breasted, open-minded.

   Syn: Unclosed; uncovered; unprotected; exposed; plain;
        apparent; obvious; evident; public; unreserved; frank;
        sincere; undissembling; artless. See {Candid}, and
        {Ingenuous}.

Chain \Chain\, n. [F. cha[^i]ne, fr. L. catena. Cf. {Catenate}.]
   1. A series of links or rings, usually of metal, connected,
      or fitted into one another, used for various purposes, as
      of support, of restraint, of ornament, of the exertion and
      transmission of mechanical power, etc.

            [They] put a chain of gold about his neck. --Dan. v.
                                                  29.

   2. That which confines, fetters, or secures, as a chain; a
      bond; as, the chains of habit.

            Driven down To chains of darkness and the undying
            worm.                                 --Milton.

   3. A series of things linked together; or a series of things
      connected and following each other in succession; as, a
      chain of mountains; a chain of events or ideas.

   4. (Surv.) An instrument which consists of links and is used
      in measuring land.

   Note: One commonly in use is Gunter's chain, which consists
         of one hundred links, each link being seven inches and
         ninety-two one hundredths in length; making up the
         total length of rods, or sixty-six, feet; hence, a
         measure of that length; hence, also, a unit for land
         measure equal to four rods square, or one tenth of an
         acre.

   5. pl. (Naut.) Iron links bolted to the side of a vessel to
      bold the dead-eyes connected with the shrouds; also, the
      channels.

   6. (Weaving) The warp threads of a web. --Knight.

   {Chain belt} (Mach.), a belt made of a chain; -- used for
      transmitting power.

   {Chain boat}, a boat fitted up for recovering lost cables,
      anchors, etc.

   {Chain bolt}
      (a) (Naut.) The bolt at the lower end of the chain plate,
          which fastens it to the vessel's side.
      (b) A bolt with a chain attached for drawing it out of
          position.

   {Chain bond}. See {Chain timber}.

   {Chain bridge}, a bridge supported by chain cables; a
      suspension bridge.

   {Chain cable}, a cable made of iron links.

   {Chain coral} (Zo["o]l.), a fossil coral of the genus
      {Halysites}, common in the middle and upper Silurian
      rocks. The tubular corallites are united side by side in
      groups, looking in an end view like links of a chain. When
      perfect, the calicles show twelve septa.

   {Chain coupling}.
      (a) A shackle for uniting lengths of chain, or connecting
          a chain with an object.
      (b) (Railroad) Supplementary coupling together of cars
          with a chain.

   {Chain gang}, a gang of convicts chained together.

   {Chain hook} (Naut.), a hook, used for dragging cables about
      the deck.

   {Chain mail}, flexible, defensive armor of hammered metal
      links wrought into the form of a garment.

   {Chain molding} (Arch.), a form of molding in imitation of a
      chain, used in the Normal style.

   {Chain pier}, a pier suspended by chain.

   {Chain pipe} (Naut.), an opening in the deck, lined with
      iron, through which the cable is passed into the lockers
      or tiers.

   {Chain plate} (Shipbuilding), one of the iron plates or
      bands, on a vessel's side, to which the standing rigging
      is fastened.

   {Chain pulley}, a pulley with depressions in the periphery of
      its wheel, or projections from it, made to fit the links
      of a chain.

   {Chain pumps}. See in the Vocabulary.

   {Chain rule} (Arith.), a theorem for solving numerical
      problems by composition of ratios, or compound proportion,
      by which, when several ratios of equality are given, the
      consequent of each being the same as the antecedent of the
      next, the relation between the first antecedent and the
      last consequent is discovered.

   {Chain shot} (Mil.), two cannon balls united by a shot chain,
      formerly used in naval warfare on account of their
      destructive effect on a ship's rigging.

   {Chain stitch}. See in the Vocabulary.

   {Chain timber}. (Arch.) See {Bond timber}, under {Bond}.

   {Chain wales}. (Naut.) Same as {Channels}.

   {Chain wheel}. See in the Vocabulary.

   {Closed chain}, {Open chain} (Chem.), terms applied to the
      chemical structure of compounds whose rational formul[ae]
      are written respectively in the form of a closed ring (see
      {Benzene nucleus}, under {Benzene}), or in an open
      extended form.

   {Endless chain}, a chain whose ends have been united by a
      link.

Source : WordNet®

open chain
     n : a chain of atoms in a molecule whose ends are not joined to
         form a ring [ant: {closed chain}]