The Collaborative International Dictionary of English v.0.48:

Flag \Flag\, n. [Cf. LG. & G. flagge, Sw. flagg, Dan. flag, D.
   vlag. See Flag to hang loose.]
   1. That which flags or hangs down loosely.
      [1913 Webster]

   2. A cloth usually bearing a device or devices and used to
      indicate nationality, party, etc., or to give or ask
      information; -- commonly attached to a staff to be waved
      by the wind; a standard; a banner; an ensign; the colors;
      as, the national flag; a military or a naval flag.
      [1913 Webster]

   3. (Zool.)
      (a) A group of feathers on the lower part of the legs of
          certain hawks, owls, etc.
      (b) A group of elongated wing feathers in certain hawks.
      (c) The bushy tail of a dog, as of a setter.
          [1913 Webster]

   4. (Zool.) One of the wing feathers next the body of a bird;
      -- called also flag feather.
      [Webster 1913 Suppl.]

   Black flag. See under Black.

   Flag captain, Flag leutenant, etc., special officers
      attached to the flagship, as aids to the flag officer.

   Flag officer, the commander of a fleet or squadron; an
      admiral, or commodore.

   Flag of truse, a white flag carried or displayed to an
      enemy, as an invitation to conference, or for the purpose
      of making some communication not hostile.

   Flag share, the flag officer's share of prize money.

   Flag station (Railroad), a station at which trains do not
      stop unless signaled to do so, by a flag hung out or
      waved.

   National flag, a flag of a particular country, on which
      some national emblem or device, is emblazoned.

   Red flag, a flag of a red color, displayed as a signal of
      danger or token of defiance; the emblem of anarchists.

   To dip, the flag, to mlower it and quickly restore it to
      its place; -- done as a mark of respect.

   To hang out the white flag, to ask truce or quarter, or, in
      some cases, to manifest a friendly design by exhibiting a
      white flag.

   To hang the flag half-mast high or To hang the flag
   half-staff or To hang the flag at half-staff, to raise it
      only half way to the mast or staff, as a token or sign of
      mourning.

   To strike the flag or To lower the flag, to haul it down,
      in token of respect, submission, or, in an engagement, of
      surrender.

   Yellow flag, the quarantine flag of all nations; also
      carried at a vessel's fore, to denote that an infectious
      disease is on board.
      [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

Quarantine \Quar"an*tine\, n. [F. quarantaine, OF. quaranteine,
   fr. F. quarante forty, L. quadraginta, akin to quattuor four,
   and E. four: cf. It. quarantina, quarentine. See Four, and
   cf. Quadragesima.]
   1. A space of forty days; -- used of Lent.
      [1913 Webster]

   2. Specifically, the term, originally of forty days, during
      which a ship arriving in port, and suspected of being
      infected a malignant contagious disease, is obliged to
      forbear all intercourse with the shore; hence, such
      restraint or inhibition of intercourse; also, the place
      where infected or prohibited vessels are stationed.
      [1913 Webster]

   Note: Quarantine is now applied also to any forced stoppage
         of travel or communication on account of malignant
         contagious disease, on land as well as by sea.
         [1913 Webster]

   3. (Eng. Law) The period of forty days during which the widow
      had the privilege of remaining in the mansion house of
      which her husband died seized.
      [1913 Webster]

   Quarantine flag, a yellow flag hoisted at the fore of a
      vessel or hung from a building, to give warning of an
      infectious disease; -- called also the yellow jack, and
      yellow flag.
      [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

Yellow \Yel"low\ (y[e^]l"l[-o]), a. [Compar. Yellower
   (y[e^]l"l[-o]*[~e]r); superl. Yellowest.] [OE. yelow,
   yelwe, [yogh]elow, [yogh]eoluw, from AS. geolu; akin to D.
   geel, OS. & OHG. gelo, G. gelb, Icel. gulr, Sw. gul, Dan.
   guul, L. helvus light bay, Gr. chlo`n young verdure, chlwro`s
   greenish yellow, Skr. hari tawny, yellowish. [root]49. Cf.
   Chlorine, Gall a bitter liquid, Gold, Yolk.]
   1. Being of a bright saffronlike color; of the color of gold
      or brass; having the hue of that part of the rainbow, or
      of the solar spectrum, which is between the orange and the
      green.
      [1913 Webster]

            Her yellow hair was browded [braided] in a tress.
                                                  --Chaucer.
      [1913 Webster]

            A sweaty reaper from his tillage brought
            First fruits, the green ear and the yellow sheaf.
                                                  --Milton.
      [1913 Webster]

            The line of yellow light dies fast away. --Keble.
      [1913 Webster]

   2. Cowardly; hence, dishonorable; mean; contemptible; as, he
      has a yellow streak. [Slang]
      [Webster 1913 Suppl.]

   3. Sensational; -- said of some newspapers, their makers,
      etc.; as, yellow journal, journalism, etc. [Colloq.]
      [Webster 1913 Suppl.]

   Yellow atrophy (Med.), a fatal affection of the liver, in
      which it undergoes fatty degeneration, and becomes rapidly
      smaller and of a deep yellow tinge. The marked symptoms
      are black vomit, delirium, convulsions, coma, and
      jaundice.

   Yellow bark, calisaya bark.

   Yellow bass (Zool.), a North American fresh-water bass
      (Morone interrupta) native of the lower parts of the
      Mississippi and its tributaries. It is yellow, with
      several more or less broken black stripes or bars. Called
      also barfish.

   Yellow berry. (Bot.) Same as Persian berry, under
      Persian.

   Yellow boy, a gold coin, as a guinea. [Slang] --Arbuthnot.

   Yellow brier. (Bot.) See under Brier.

   Yellow bugle (Bot.), a European labiate plant (Ajuga
      Chamaepitys).

   Yellow bunting (Zool.), the European yellow-hammer.

   Yellow cat (Zool.), a yellow catfish; especially, the
      bashaw.

   Yellow copperas (Min.), a hydrous sulphate of iron; --
      called also copiapite.

   Yellow copper ore, a sulphide of copper and iron; copper
      pyrites. See Chalcopyrite.

   Yellow cress (Bot.), a yellow-flowered, cruciferous plant
      (Barbarea praecox), sometimes grown as a salad plant.

   Yellow dock. (Bot.) See the Note under Dock.

   Yellow earth, a yellowish clay, colored by iron, sometimes
      used as a yellow pigment.

   Yellow fever (Med.), a malignant, contagious, febrile
      disease of warm climates, attended with jaundice,
      producing a yellow color of the skin, and with the black
      vomit. See Black vomit, in the Vocabulary.

   Yellow flag, the quarantine flag. See under Quarantine,
      and 3d Flag.

   Yellow jack.
      (a) The yellow fever. See under 2d Jack.
      (b) The quarantine flag. See under Quarantine.

   Yellow jacket (Zool.), any one of several species of
      American social wasps of the genus Vespa, in which the
      color of the body is partly bright yellow. These wasps are
      noted for their irritability, and for their painful
      stings.

   Yellow lead ore (Min.), wulfenite.

   Yellow lemur (Zool.), the kinkajou.

   Yellow macauco (Zool.), the kinkajou.

   Yellow mackerel (Zool.), the jurel.

   Yellow metal. Same as Muntz metal, under Metal.

   Yellow ocher (Min.), an impure, earthy variety of brown
      iron ore, which is used as a pigment.

   Yellow oxeye (Bot.), a yellow-flowered plant
      (Chrysanthemum segetum) closely related to the oxeye
      daisy.

   Yellow perch (Zool.), the common American perch. See
      Perch.

   Yellow pike (Zool.), the wall-eye.

   Yellow pine (Bot.), any of several kinds of pine; also,
      their yellowish and generally durable timber. Among the
      most common are valuable species are Pinus mitis and
      Pinus palustris of the Eastern and Southern States, and
      Pinus ponderosa and Pinus Arizonica of the Rocky
      Mountains and Pacific States.

   Yellow plover (Zool.), the golden plover.

   Yellow precipitate (Med. Chem.), an oxide of mercury which
      is thrown down as an amorphous yellow powder on adding
      corrosive sublimate to limewater.

   Yellow puccoon. (Bot.) Same as Orangeroot.

   Yellow rail (Zool.), a small American rail (Porzana
      Noveboracensis) in which the lower parts are dull yellow,
      darkest on the breast. The back is streaked with brownish
      yellow and with black, and spotted with white. Called also
      yellow crake.

   Yellow rattle, Yellow rocket. (Bot.) See under Rattle,
      and Rocket.

   Yellow Sally (Zool.), a greenish or yellowish European
      stone fly of the genus Chloroperla; -- so called by
      anglers.

   Yellow sculpin (Zool.), the dragonet.

   Yellow snake (Zool.), a West Indian boa (Chilobothrus
      inornatus) common in Jamaica. It becomes from eight to
      ten long. The body is yellowish or yellowish green, mixed
      with black, and anteriorly with black lines.

   Yellow spot.
      (a) (Anat.) A small yellowish spot with a central pit, the
          fovea centralis, in the center of the retina where
          vision is most accurate. See Eye.
      (b) (Zool.) A small American butterfly (Polites Peckius)
          of the Skipper family. Its wings are brownish, with a
          large, irregular, bright yellow spot on each of the
          hind wings, most conspicuous beneath. Called also
          Peck's skipper. See Illust. under Skipper, n., 5.


   Yellow tit (Zool.), any one of several species of crested
      titmice of the genus Machlolophus, native of India. The
      predominating colors of the plumage are yellow and green.


   Yellow viper (Zool.), the fer-de-lance.

   Yellow warbler (Zool.), any one of several species of
      American warblers of the genus Dendroica in which the
      predominant color is yellow, especially Dendroica
      aestiva, which is a very abundant and familiar species;
      -- called also garden warbler, golden warbler, summer
      yellowbird, summer warbler, and yellow-poll warbler.


   Yellow wash (Pharm.), yellow oxide of mercury suspended in
      water, -- a mixture prepared by adding corrosive sublimate
      to limewater.

   Yellow wren (Zool.)
      (a) The European willow warbler.
      (b) The European wood warbler.
          [1913 Webster]

WordNet (r) 3.0 (2006):

yellow flag
    n 1: common yellow-flowered iris of Europe and North Africa,
         naturalized in United States and often cultivated [syn:
         yellow iris, yellow flag, yellow water flag, Iris
         pseudacorus]