The Collaborative International Dictionary of English v.0.48:

Transit \Trans"it\, n. [L. transitus, from transire to go over:
   cf. F. transit. See Transient.]
   1. The act of passing; passage through or over.
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            In France you are now . . . in the transit from one
            form of government to another.        --Burke.
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   2. The act or process of causing to pass; conveyance; as, the
      transit of goods through a country.
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   3. A line or route of passage or conveyance; as, the
      Nicaragua transit. --E. G. Squier.
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   4. (Astron.)
      (a) The passage of a heavenly body over the meridian of a
          place, or through the field of a telescope.
      (b) The passage of a smaller body across the disk of a
          larger, as of Venus across the sun's disk, or of a
          satellite or its shadow across the disk of its
          primary.
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   5. An instrument resembling a theodolite, used by surveyors
      and engineers; -- called also transit compass, and
      surveyor's transit.
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   Note: The surveyor's transit differs from the theodolite in
         having the horizontal axis attached directly to the
         telescope which is not mounted in Y's and can be turned
         completely over about the axis.
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   Lower transit (Astron.), the passage of a heavenly body
      across that part of the meridian which is below the polar
      axis.

   Surveyor's transit. See Transit, 5, above.

   Transit circle (Astron.), a transit instrument with a
      graduated circle attached, used for observing the time of
      transit and the declination at one observation. See
      Circle, n., 3.

   Transit compass. See Transit, 5, above.

   Transit duty, a duty paid on goods that pass through a
      country.

   Transit instrument. (Astron.)
      (a) A telescope mounted at right angles to a horizontal
          axis, on which it revolves with its line of
          collimation in the plane of the meridian, -- used in
          connection with a clock for observing the time of
          transit of a heavenly body over the meridian of a
          place.
      (b) (Surv.) A surveyor's transit. See Transit, 5, above.


   Transit trade (Com.), the business conected with the
      passage of goods through a country to their destination.


   Upper transit (Astron.), the passage of a heavenly body
      across that part of the meridian which is above the polar
      axis.
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WordNet (r) 3.0 (2006):

transit instrument
    n 1: a telescope mounted on an axis running east and west and
         used to time the transit of a celestial body across the
         meridian