The Collaborative International Dictionary of English v.0.48:

Straight \Straight\, a. [Compar. Straighter; superl.
   Straightest.] [OE. strei?t, properly p. p. of strecchen to
   stretch, AS. streht, p. p. of streccan to stretch, to extend.
   See Stretch.]
   1. Right, in a mathematical sense; passing from one point to
      another by the nearest course; direct; not deviating or
      crooked; as, a straight line or course; a straight piece
      of timber.
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            And the crooked shall be made straight. --Isa. xl.
                                                  4.
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            There are many several sorts of crooked lines, but
            there is only one which is straight.  --Dryden.
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   2. (Bot.) Approximately straight; not much curved; as,
      straight ribs are such as pass from the base of a leaf to
      the apex, with a small curve.
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   3. (Card Playing) Composed of cards which constitute a
      regular sequence, as the ace, king, queen, jack, and
      ten-spot; as, a straight hand; a straight flush.
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   4. Conforming to justice and rectitude; not deviating from
      truth or fairness; upright; as, straight dealing.
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   5. Unmixed; undiluted; as, to take liquor straight. [Slang]
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   6. Making no exceptions or deviations in one's support of the
      organization and candidates of a political party; as, a
      straight Republican; a straight Democrat; also, containing
      the names of all the regularly nominated candidates of a
      party and no others; as, a straight ballot. [Political
      Cant, U.S.]
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   Straight arch (Arch.), a form of arch in which the intrados
      is straight, but with its joints drawn radially, as in a
      common arch.

   A straight face, one giving no evidence of merriment or
      other emotion.

   A straight line. "That which lies evenly between its
      extreme points." --Euclid. "The shortest line between two
      points." --Chauvenet. "A line which has the same direction
      through its whole length." --Newcomb.

   Straight-way valve, a valve which, when opened widely,
      affords a straight passageway, as for water.
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WordNet (r) 3.0 (2006):

straight arch
    n 1: an arch with mutually supporting voussoirs that has a
         straight horizontal extrados and intrados [syn: flat
         arch, straight arch]