Source : Webster's Revised Unabridged Dictionary (1913)

Shaft \Shaft\, n. [OE. shaft, schaft, AS. sceaft; akin to D.
   schacht, OHG. scaft, G. schaft, Dan. & Sw. skaft handle,
   haft, Icel. skapt, and probably to L. scapus, Gr. ????, ????,
   a staff. Probably originally, a shaven or smoothed rod. Cf.
   {Scape}, {Scepter}, {Shave}.]
   1. The slender, smooth stem of an arrow; hence, an arrow.

            His sleep, his meat, his drink, is him bereft, That
            lean he wax, and dry as is a shaft.   --Chaucer.

            A shaft hath three principal parts, the stele
            [stale], the feathers, and the head.  --Ascham.

   2. The long handle of a spear or similar weapon; hence, the
      weapon itself; (Fig.) anything regarded as a shaft to be
      thrown or darted; as, shafts of light.

            And the thunder, Winged with red lightning and
            impetuous rage, Perhaps hath spent his shafts.
                                                  --Milton.

            Some kinds of literary pursuits . . . have been
            attacked with all the shafts of ridicule. --V. Knox.

   3. That which resembles in some degree the stem or handle of
      an arrow or a spear; a long, slender part, especially when
      cylindrical. Specifically: (a) (Bot.) The trunk, stem, or
      stalk of a plant.
      (b) (Zo["o]l.) The stem or midrib of a feather. See
          Illust. of {Feather}.
      (c) The pole, or tongue, of a vehicle; also, a thill.
      (d) The part of a candlestick which supports its branches.

                Thou shalt make a candlestick of pure gold . . .
                his shaft, and his branches, his bowls, his
                knops, and his flowers, shall be of the same.
                                                  --Ex. xxv. 31.
      (e) The handle or helve of certain tools, instruments,
          etc., as a hammer, a whip, etc.
      (f) A pole, especially a Maypole. [Obs.] --Stow.
      (g) (Arch.) The body of a column; the cylindrical pillar
          between the capital and base (see Illust. of
          {Column}). Also, the part of a chimney above the roof.
          Also, the spire of a steeple. [Obs. or R.] --Gwilt.
      (h) A column, an obelisk, or other spire-shaped or
          columnar monument.

                Bid time and nature gently spare The shaft we
                raise to thee.                    --Emerson.
      (i) (Weaving) A rod at the end of a heddle.
      (j) (Mach.) A solid or hollow cylinder or bar, having one
          or more journals on which it rests and revolves, and
          intended to carry one or more wheels or other
          revolving parts and to transmit power or motion; as,
          the shaft of a steam engine. See Illust. of
          {Countershaft}.

   4. (Zo["o]l.) A humming bird ({Thaumastura cora}) having two
      of the tail feathers next to the middle ones very long in
      the male; -- called also {cora humming bird}.

   5. [Cf. G. schacht.] (Mining) A well-like excavation in the
      earth, perpendicular or nearly so, made for reaching and
      raising ore, for raising water, etc.

   6. A long passage for the admission or outlet of air; an air
      shaft.

   7. The chamber of a blast furnace.

   {Line shaft} (Mach.), a main shaft of considerable length, in
      a shop or factory, usually bearing a number of pulleys by
      which machines are driven, commonly by means of
      countershafts; -- called also {line}, or {main line}.

   {Shaft alley} (Naut.), a passage extending from the engine
      room to the stern, and containing the propeller shaft.

   {Shaft furnace} (Metal.), a furnace, in the form of a
      chimney, which is charged at the top and tapped at the
      bottom.

Source : WordNet®

main line
     n : the principal route of a transportation system