The Collaborative International Dictionary of English v.0.48:

Circular \Cir"cu*lar\, a. [L. circularis, fr. circulus circle:
   cf. F. circulaire. See Circle.]
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   1. In the form of, or bounded by, a circle; round.
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   2. repeating itself; ending in itself; reverting to the point
      of beginning; hence, illogical; inconclusive; as, circular
      reasoning.
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   3. Adhering to a fixed circle of legends; cyclic; hence,
      mean; inferior. See Cyclic poets, under Cyclic.
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            Had Virgil been a circular poet, and closely adhered
            to history, how could the Romans have had Dido?
                                                  --Dennis.
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   4. Addressed to a circle, or to a number of persons having a
      common interest; circulated, or intended for circulation;
      as, a circular letter.
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            A proclamation of Henry III., . . . doubtless
            circular throughout England.          --Hallam.
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   5. Perfect; complete. [Obs.]
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            A man so absolute and circular
            In all those wished-for rarities that may take
            A virgin captive.                     --Massinger.
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   Circular are, any portion of the circumference of a circle.


   Circular cubics (Math.), curves of the third order which
      are imagined to pass through the two circular points at
      infinity.

   Circular functions. (Math.) See under Function.

   Circular instruments, mathematical instruments employed for
      measuring angles, in which the graduation extends round
      the whole circumference of a circle, or 360[deg].

   Circular lines, straight lines pertaining to the circle, as
      sines, tangents, secants, etc.

   Circular note or Circular letter.
      (a) (Com.) See under Credit.
      (b) (Diplomacy) A letter addressed in identical terms to a
          number of persons.

   Circular numbers (Arith.), those whose powers terminate in
      the same digits as the roots themselves; as 5 and 6, whose
      squares are 25 and 36. --Bailey. --Barlow.

   Circular points at infinity (Geom.), two imaginary points
      at infinite distance through which every circle in the
      plane is, in the theory of curves, imagined to pass.

   Circular polarization. (Min.) See under Polarization.

   Circular sailing or Globular sailing (Naut.), the method
      of sailing by the arc of a great circle.

   Circular saw. See under Saw.
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