Source : Webster's Revised Unabridged Dictionary (1913)

Lag \Lag\, n.
   1. One who lags; that which comes in last. [Obs.] ``The lag
      of all the flock.'' --Pope.

   2. The fag-end; the rump; hence, the lowest class.

            The common lag of people.             --Shak.

   3. The amount of retardation of anything, as of a valve in a
      steam engine, in opening or closing.

   4. A stave of a cask, drum, etc.; especially (Mach.), one of
      the narrow boards or staves forming the covering of a
      cylindrical object, as a boiler, or the cylinder of a
      carding machine or a steam engine.

   5. (Zo["o]l.) See {Graylag}.

   {Lag of the tide}, the interval by which the time of high
      water falls behind the mean time, in the first and third
      quarters of the moon; -- opposed to {priming} of the tide,
      or the acceleration of the time of high water, in the
      second and fourth quarters; depending on the relative
      positions of the sun and moon.

   {Lag screw}, an iron bolt with a square head, a sharp-edged
      thread, and a sharp point, adapted for screwing into wood;
      a screw for fastening lags.

Screw \Screw\ (skr[udd]), n. [OE. scrue, OF. escroue, escroe,
   female screw, F. ['e]crou, L. scrobis a ditch, trench, in
   LL., the hole made by swine in rooting; cf. D. schroef a
   screw, G. schraube, Icel. skr[=u]fa.]
   1. A cylinder, or a cylindrical perforation, having a
      continuous rib, called the thread, winding round it
      spirally at a constant inclination, so as to leave a
      continuous spiral groove between one turn and the next, --
      used chiefly for producing, when revolved, motion or
      pressure in the direction of its axis, by the sliding of
      the threads of the cylinder in the grooves between the
      threads of the perforation adapted to it, the former being
      distinguished as the external, or male screw, or, more
      usually the screw; the latter as the internal, or female
      screw, or, more usually, the nut.

   Note: The screw, as a mechanical power, is a modification of
         the inclined plane, and may be regarded as a
         right-angled triangle wrapped round a cylinder, the
         hypotenuse of the marking the spiral thread of the
         screw, its base equaling the circumference of the
         cylinder, and its height the pitch of the thread.

   2. Specifically, a kind of nail with a spiral thread and a
      head with a nick to receive the end of the screw-driver.
      Screws are much used to hold together pieces of wood or to
      fasten something; -- called also {wood screws}, and {screw
      nails}. See also {Screw bolt}, below.

   3. Anything shaped or acting like a screw; esp., a form of
      wheel for propelling steam vessels. It is placed at the
      stern, and furnished with blades having helicoidal
      surfaces to act against the water in the manner of a
      screw. See {Screw propeller}, below.

   4. A steam vesel propelled by a screw instead of wheels; a
      screw steamer; a propeller.

   5. An extortioner; a sharp bargainer; a skinflint; a niggard.
      --Thackeray.

   6. An instructor who examines with great or unnecessary
      severity; also, a searching or strict examination of a
      student by an instructor. [Cant, American Colleges]

   7. A small packet of tobacco. [Slang] --Mayhew.

   8. An unsound or worn-out horse, useful as a hack, and
      commonly of good appearance. --Ld. Lytton.

   9. (Math.) A straight line in space with which a definite
      linear magnitude termed the pitch is associated (cf. 5th
      {Pitch}, 10
      (b) ). It is used to express the displacement of a rigid
          body, which may always be made to consist of a
          rotation about an axis combined with a translation
          parallel to that axis.

   10. (Zo["o]l.) An amphipod crustacean; as, the skeleton screw
       ({Caprella}). See {Sand screw}, under {Sand}.

   {Archimedes screw}, {Compound screw}, {Foot screw}, etc. See
      under {Archimedes}, {Compound}, {Foot}, etc.

   {A screw loose}, something out of order, so that work is not
      done smoothly; as, there is a screw loose somewhere. --H.
      Martineau.

   {Endless, or perpetual, {screw}, a screw used to give motion
      to a toothed wheel by the action of its threads between
      the teeth of the wheel; -- called also a {worm}.

   {Lag screw}. See under {Lag}.

   {Micrometer screw}, a screw with fine threads, used for the
      measurement of very small spaces.

   {Right and left screw}, a screw having threads upon the
      opposite ends which wind in opposite directions.

   {Screw alley}. See {Shaft alley}, under {Shaft}.

   {Screw bean}. (Bot.)
       (a) The curious spirally coiled pod of a leguminous tree
           ({Prosopis pubescens}) growing from Texas to
           California. It is used for fodder, and ground into
           meal by the Indians.
       (b) The tree itself. Its heavy hard wood is used for
           fuel, for fencing, and for railroad ties.

   {Screw bolt}, a bolt having a screw thread on its shank, in
      distinction from a {key bolt}. See 1st {Bolt}, 3.

   {Screw box}, a device, resembling a die, for cutting the
      thread on a wooden screw.

   {Screw dock}. See under {Dock}.

   {Screw engine}, a marine engine for driving a screw
      propeller.

   {Screw gear}. See {Spiral gear}, under {Spiral}.

   {Screw jack}. Same as {Jackscrew}.

   {Screw key}, a wrench for turning a screw or nut; a spanner
      wrench.

   {Screw machine}.
       (a) One of a series of machines employed in the
           manufacture of wood screws.
       (b) A machine tool resembling a lathe, having a number of
           cutting tools that can be caused to act on the work
           successively, for making screws and other turned
           pieces from metal rods.

   {Screw pine} (Bot.), any plant of the endogenous genus
      {Pandanus}, of which there are about fifty species,
      natives of tropical lands from Africa to Polynesia; --
      named from the spiral arrangement of the pineapple-like
      leaves.

   {Screw plate}, a device for cutting threads on small screws,
      consisting of a thin steel plate having a series of
      perforations with internal screws forming dies.

   {Screw press}, a press in which pressure is exerted by means
      of a screw.

   {Screw propeller}, a screw or spiral bladed wheel, used in
      the propulsion of steam vessels; also, a steam vessel
      propelled by a screw.

   {Screw shell} (Zo["o]l.), a long, slender, spiral gastropod
      shell, especially of the genus Turritella and allied
      genera. See {Turritella}.

   {Screw steamer}, a steamship propelled by a screw.

   {Screw thread}, the spiral rib which forms a screw.

   {Screw stone} (Paleon.), the fossil stem of an encrinite.

   {Screw tree} (Bot.), any plant of the genus {Helicteres},
      consisting of about thirty species of tropical shrubs,
      with simple leaves and spirally twisted, five-celled
      capsules; -- also called {twisted-horn}, and {twisty}.

   {Screw valve}, a stop valve which is opened or closed by a
      screw.

   {Screw worm} (Zo["o]l.), the larva of an American fly
      ({Compsomyia macellaria}), allied to the blowflies, which
      sometimes deposits its eggs in the nostrils, or about
      wounds, in man and other animals, with fatal results.

   {Screw wrench}.
       (a) A wrench for turning a screw.
       (b) A wrench with an adjustable jaw that is moved by a
           screw.

   {To put the} {screw, or screws}, {on}, to use pressure upon,
      as for the purpose of extortion; to coerce.

   {To put under the} {screw or screws}, to subject to pressure;
      to force.

   {Wood screw}, a metal screw with a sharp thread of coarse
      pitch, adapted to holding fast in wood. See Illust. of
      {Wood screw}, under {Wood}.

Source : WordNet®

lag screw
     n : a heavy woodscrew with a square or hexagonal head that is
         driven in with a wrench [syn: {lag bolt}]