The Collaborative International Dictionary of English v.0.48:

Cardinal \Car"di*nal\, a. [L. cardinalis, fr. cardo the hinge of
   a door, that on which a thing turns or depends: cf. F.
   cardinal.]
   Of fundamental importance; pre["e]minent; superior; chief;
   principal.
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         The cardinal intersections of the zodiac. --Sir T.
                                                  Browne.
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         Impudence is now a cardinal virtue.      --Drayton.
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         But cardinal sins, and hollow hearts, I fear ye.
                                                  --Shak.
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   Cardinal numbers, the numbers one, two, three, etc., in
      distinction from first, second, third, etc., which are
      called ordinal numbers.

   Cardinal points
   (a) (Geol.) The four principal points of the compass, or
       intersections of the horizon with the meridian and the
       prime vertical circle, north, south east, and west.
   (b) (Astrol.) The rising and setting of the sun, the zenith
       and nadir.

   Cardinal signs (Astron.) Aries, Libra, Cancer, and
      Capricorn.

   Cardinal teeth (Zool.), the central teeth of bivalve shell.
      See Bivalve.

   Cardinal veins (Anat.), the veins in vertebrate embryos,
      which run each side of the vertebral column and returm the
      blood to the heart. They remain through life in some
      fishes.

   Cardinal virtues, pre["e]minent virtues; among the
      ancients, prudence, justice, temperance, and fortitude.

   Cardinal winds, winds which blow from the cardinal points
      due north, south, east, or west.
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The Collaborative International Dictionary of English v.0.48:

Cardinal \Car"di*nal\, n. [F. carinal, It. cardinale, LL.
   cardinalis (ecclesi[ae] Roman[ae]). See Cardinal, a.]
   1. (R. C. Ch.) One of the ecclesiastical princes who
      constitute the pope's council, or the sacred college.
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            The clerics of the supreme Chair are called
            Cardinals, as undoubtedly adhering more nearly to
            the hinge by which all things are moved. --Pope Leo
                                                  IX.
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   Note: The cardinals are appointed by the pope. Since the time
         of Sixtus V., their number can never exceed seventy
         (six of episcopal rank, fifty priests, fourteen
         deacons), and the number of cardinal priests and
         deacons is seldom full. When the papel chair is vacant
         a pope is elected by the college of cardinals from
         among themselves. The cardinals take precedence of all
         dignitaries except the pope. The principal parts of a
         cardinal's costume are a red cassock, a rochet, a short
         purple mantle, and a red hat with a small crown and
         broad brim, with cords and tessels of a special pattern
         hanging from it.
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   2. A woman's short cloak with a hood.
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            Where's your cardinal! Make haste.    --Lloyd.
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   3. Mulled red wine. --Hotten.
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   4. the cardinal bird, also called the northern cardinal.
      [PJC]

   Cardinal bird, or Cardinal grosbeak (Zool.), an American
      song bird (Cardinalis cardinalis, or Cardinalis
      Virginianus), of the family Fringillid[ae], or finches
      of which the male has a bright red plumage, and both sexes
      have a high, pointed crest on its head; -- it is also
      called the northern cardinal or eastern cardinal. The
      males have loud and musical notes resembling those of a
      fife. Other related species are also called cardinal
      birds.

   Cardinal flower (Bot.), an herbaceous plant (Lobelia
      cardinalis) bearing brilliant red flowers of much beauty.


   Cardinal red, a color like that of a cardinal's cassock,
      hat, etc.; a bright red, darker than scarlet, and between
      scarlet and crimson.
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WordNet (r) 3.0 (2006):

cardinal
    adj 1: serving as an essential component; "a cardinal rule";
           "the central cause of the problem"; "an example that was
           fundamental to the argument"; "computers are fundamental
           to modern industrial structure" [syn: cardinal,
           central, fundamental, key, primal]
    2: being or denoting a numerical quantity but not order;
       "cardinal numbers" [ant: ordinal]
    n 1: (Roman Catholic Church) one of a group of more than 100
         prominent bishops in the Sacred College who advise the Pope
         and elect new Popes
    2: the number of elements in a mathematical set; denotes a
       quantity but not the order [syn: cardinal number,
       cardinal]
    3: a variable color averaging a vivid red [syn: cardinal,
       carmine]
    4: crested thick-billed North American finch having bright red
       plumage in the male [syn: cardinal, cardinal grosbeak,
       Richmondena Cardinalis, Cardinalis cardinalis, redbird]

Moby Thesaurus II by Grady Ward, 1.0:

213 Moby Thesaurus words for "cardinal":
   Gaussian integer, Grand Penitentiary, Holy Father, Titian,
   Titian-red, abuna, algebraic number, algorismic, algorithmic,
   aliquot, all-absorbing, antipope, arch, archbishop, archdeacon,
   archpriest, banner, bishop, bishop coadjutor, bricky, canon,
   capital, cardinal bishop, cardinal deacon, cardinal number,
   cardinal priest, carmine, carnation, carnelian, central, cerise,
   champion, chaplain, cherry, cherry-colored, cherry-red, chief,
   coadjutor, complex number, constitutive, controlling, crimson,
   crowning, curate, damask, dean, decimal, defective number,
   differential, digital, diocesan, dominant, ecclesiarch, essential,
   even, even number, exarch, exponential, ferruginous, fiery,
   figural, figurate, figurative, finite, finite number, fire-red,
   first, flame-colored, flame-red, flaming, focal, foremost,
   fraction, fractional, fundamental, glowing, great, gules, headmost,
   hegemonic, hierarch, high priest, highest, hot, imaginary,
   imaginary number, impair, important, impossible, incarmined,
   infinite, infinity, inflamed, infrared, integer, integral,
   iron-red, irrational, irrational number, key, lake-colored, laky,
   lateritious, leading, lobster-red, logarithmic, logometric, lurid,
   magisterial, main, maroon, master, metropolitan, mixed number,
   necessary, negative, numeral, numerary, numerative, numeric, odd,
   ordinal, overriding, overruling, pair, papa, paramount, patriarch,
   penitentiary, pivotal, polygonal number, pontiff, pope, port-wine,
   positive, possible, prebendary, predominant, preeminent, prelate,
   premier, preponderant, prevailing, primal, primary, primate, prime,
   prime number, principal, puce, pure imaginary, radical, ranking,
   rational, rational number, real, real number, reciprocal,
   rectangular number, rector, red, red-dyed, red-looking, reddened,
   reddish, reddish-amber, reddish-brown, round number, rubicund,
   rubiginous, rubric, rubricose, ruby, ruby-colored, ruby-red,
   ruddied, ruddy, rufescent, rufous, ruling, rural dean, rust,
   rust-red, rusty, scarlet, serial number, sovereign, special,
   stammel, star, stellar, subdean, submultiple, suffragan,
   supereminent, supreme, surd, tile-red, topflight, topmost,
   transcendental, transcendental number, transfinite number,
   uppermost, vermilion, vicar, vinaceous, vital, warm, whole number,
   wine, wine-colored, wine-red



Bouvier's Law Dictionary, Revised 6th Ed (1856):

CARDINAL, eccl. law. The title given to one of the highest dignitaries of
the court of Rome. Cardinals are next to the pope in dignity; he is elected
by them and out of their body. There are cardinal bishops, cardinal priests,
and cardinal deacons. See Fleury, Hist. Eccles. liv. xxxv. n. 17, II. n. 19
Thomassin, part ii. liv. i. oh. 53, part iv. liv. i. c. 79, 80 Loiseau,
Traite des Ordres, c. 3, n. 31; Andre, Droit Canon, au mot.