The Collaborative International Dictionary of English v.0.48:

Thistle \This"tle\, n. [OE. thistil, AS. [thorn]istel; akin to
   D. & G. distel, OHG. distila, distil, Icel. [thorn]istill,
   Sw. tistel, Dan. tidsel; of uncertain origin.] (Bot.)
   Any one of several prickly composite plants, especially those
   of the genera Cnicus, Craduus, and Onopordon. The name
   is often also applied to other prickly plants.
   [1913 Webster]

   Blessed thistle, Carduus benedictus, so named because it
      was formerly considered an antidote to the bite of
      venomous creatures.

   Bull thistle, Cnicus lanceolatus, the common large
      thistle of neglected pastures.

   Canada thistle, Cnicus arvensis, a native of Europe, but
      introduced into the United States from Canada.

   Cotton thistle, Onopordon Acanthium.

   Fuller's thistle, the teasel.

   Globe thistle, Melon thistle, etc. See under Globe,
      Melon, etc.

   Pine thistle, Atractylis gummifera, a native of the
      Mediterranean region. A vicid gum resin flows from the
      involucre.

   Scotch thistle, either the cotton thistle, or the musk
      thistle, or the spear thistle; -- all used national
      emblems of Scotland.

   Sow thistle, Sonchus oleraceus.

   Spear thistle. Same as Bull thistle.

   Star thistle, a species of Centaurea. See Centaurea.

   Torch thistle, a candelabra-shaped plant of the genus
      Cereus. See Cereus.

   Yellow thistle, Cincus horridulus.
      [1913 Webster]

   Thistle bird (Zool.), the American goldfinch, or
      yellow-bird (Spinus tristis); -- so called on account of
      its feeding on the seeds of thistles. See Illust. under
      Goldfinch.

   Thistle butterfly (Zool.), a handsomely colored American
      butterfly (Vanessa cardui) whose larva feeds upon
      thistles; -- called also painted lady.

   Thistle cock (Zool.), the corn bunting (Emberiza
      militaria). [Prov. Eng.]

   Thistle crown, a gold coin of England of the reign of James
      I., worth four shillings.

   Thistle finch (Zool.), the goldfinch; -- so called from its
      fondness for thistle seeds. [Prov. Eng.]

   Thistle funnel, a funnel having a bulging body and flaring
      mouth.
      [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

Cotton \Cot"ton\ (k[o^]t"t'n), n. [F. coton, Sp. algodon the
   cotton plant and its wool, coton printed cotton, cloth, fr.
   Ar. qutun, alqutun, cotton wool. Cf. Acton, Hacqueton.]
   1. A soft, downy substance, resembling fine wool, consisting
      of the unicellular twisted hairs which grow on the seeds
      of the cotton plant. Long-staple cotton has a fiber
      sometimes almost two inches long; short-staple, from two
      thirds of an inch to an inch and a half.
      [1913 Webster]

   2. The cotton plant. See Cotten plant, below.
      [1913 Webster]

   3. Cloth made of cotton.
      [1913 Webster]

   Note: Cotton is used as an adjective before many nouns in a
         sense which commonly needs no explanation; as, cotton
         bagging; cotton cloth; cotton goods; cotton industry;
         cotton mill; cotton spinning; cotton tick.
         [1913 Webster]

   Cotton cambric. See Cambric, n., 2.

   Cotton flannel, the manufactures' name for a heavy cotton
      fabric, twilled, and with a long plush nap. In England it
      is called swan's-down cotton, or Canton flannel.

   Cotton gin, a machine to separate the seeds from cotton,
      invented by Eli Whitney.

   Cotton grass (Bot.), a genus of plants (Eriphorum) of the
      Sedge family, having delicate capillary bristles
      surrounding the fruit (seedlike achenia), which elongate
      at maturity and resemble tufts of cotton.

   Cotton mouse (Zool.), a field mouse (Hesperomys
      gossypinus), injurious to cotton crops.

   Cotton plant (Bot.), a plant of the genus Gossypium, of
      several species, all growing in warm climates, and bearing
      the cotton of commerce. The common species, originally
      Asiatic, is Gossypium herbaceum.

   Cotton press, a building and machinery in which cotton
      bales are compressed into smaller bulk for shipment; a
      press for baling cotton.

   Cotton rose (Bot.), a genus of composite herbs (Filago),
      covered with a white substance resembling cotton.

   Cotton scale (Zool.), a species of bark louse (Pulvinaria
      innumerabilis), which does great damage to the cotton
      plant.

   Cotton shrub. Same as Cotton plant.

   Cotton stainer (Zool.), a species of hemipterous insect
      (Dysdercus suturellus), which seriously damages growing
      cotton by staining it; -- called also redbug.

   Cotton thistle (Bot.), the Scotch thistle. See under
      Thistle.

   Cotton velvet, velvet in which the warp and woof are both
      of cotton, and the pile is of silk; also, velvet made
      wholly of cotton.

   Cotton waste, the refuse of cotton mills.

   Cotton wool, cotton in its raw or woolly state.

   Cotton worm (Zool.), a lepidopterous insect (Aletia
      argillacea), which in the larval state does great damage
      to the cotton plant by eating the leaves. It also feeds on
      corn, etc., and hence is often called corn worm, and
      Southern army worm.
      [1913 Webster]

WordNet (r) 3.0 (2006):

cotton thistle
    n 1: biennial Eurasian white hairy thistle having pale purple
         flowers; naturalized in North America [syn: cotton
         thistle, woolly thistle, Scotch thistle, Onopordum
         acanthium, Onopordon acanthium]