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}.

Source : WordNet®

open circuit
     n : an incomplete electrical circuit in which no current flows
         [ant: {closed circuit}]