Source : Webster's Revised Unabridged Dictionary (1913)

Umbrella \Um*brel"la\, n. [It. umbrella, fr. ombra a shade, L.
   umbra; cf. L. umbella a sunshade, a parasol. Cf. {Umbel},
   {Umbrage}.]
   1. A shade, screen, or guard, carried in the hand for
      sheltering the person from the rays of the sun, or from
      rain or snow. It is formed of silk, cotton, or other
      fabric, extended on strips of whalebone, steel, or other
      elastic material, inserted, or fastened to, a rod or stick
      by means of pivots or hinges, in such a way as to allow of
      being opened and closed with ease. See {Parasol}.

            Underneath the umbrella's oily shed.  --Gay.

   2. (Zo["o]l.) The umbrellalike disk, or swimming bell, of a
      jellyfish.

   3. (Zo["o]l.) Any marine tectibranchiate gastropod of the
      genus {Umbrella}, having an umbrella-shaped shell; --
      called also {umbrella shell}.

   {Umbrella ant} (Zo["o]l.), the sauba ant; -- so called
      because it carries bits of leaves over its back when
      foraging. Called also {parasol ant}.

   {Umbrella bird} (Zo["o]l.), a South American bird
      ({Cephalopterus ornatus}) of the family {Cotingid[ae]}. It
      is black, with a large handsome crest consisting of a mass
      of soft, glossy blue feathers curved outward at the tips.
      It also has a cervical plume consisting of a long,
      cylindrical dermal process covered with soft hairy
      feathers. Called also {dragoon bird}.

   {Umbrella leaf} (Bot.), an American perennial herb
      ({Dyphylleia cymosa}), having very large peltate and lobed
      radical leaves.

   {Umbrella shell}. (Zo["o]l.) See {Umbrella}, 3.

   {Umbrella tree} (Bot.), a kind of magnolia ({M. Umbrella})
      with the large leaves arranged in umbrellalike clusters at
      the ends of the branches. It is a native of Pennsylvania,
      Virginia, and Kentucky. Other plants in various countries
      are called by this name, especially a kind of screw pine
      ({Pandanus odoratissimus}).

Source : WordNet®

umbrella
     adj : covering or applying simultaneously to a number of similar
           items or elements or groups; "an umbrella
           organization"; "umbrella insurance coverage"

umbrella
     n 1: a lightweight handheld collapsible canopy
     2: a formation of military planes maintained over ground
        operations or targets; "an air umbrella over England"
     3: having the function of uniting a group of similar things;
        "the Democratic Party is an umbrella for many liberal
        groups"; "under the umbrella of capitalism"