The Collaborative International Dictionary of English v.0.48:

Nettle \Net"tle\, n. [AS. netele; akin to D. netel, G. nessel,
   OHG. nezz["i]la, nazza, Dan. nelde, n[aum]lde, Sw.
   n[aum]ssla; cf, Lith. notere.] (Bot.)
   A plant of the genus Urtica, covered with minute sharp
   hairs containing a poison that produces a stinging sensation.
   Urtica gracilis is common in the Northern, and Urtica
   chamaedryoides in the Southern, United States. The common
   European species, Urtica urens and Urtica dioica, are
   also found in the Eastern united States. Urtica pilulifera
   is the Roman nettle of England.
   [1913 Webster]

   Note: The term nettle has been given to many plants related
         to, or to some way resembling, the true nettle; as:

   Australian nettle, a stinging tree or shrub of the genus
      Laportea (as Laportea gigas and Laportea moroides);
      -- also called nettle tree.

   Bee nettle, Hemp nettle, a species of Galeopsis. See
      under Hemp.

   Blind nettle, Dead nettle, a harmless species of
      Lamium.

   False nettle (Baehmeria cylindrica), a plant common in
      the United States, and related to the true nettles.

   Hedge nettle, a species of Stachys. See under Hedge.

   Horse nettle (Solanum Carolinense). See under Horse.

   nettle tree.
   (a) Same as Hackberry.
   (b) See Australian nettle (above).

   Spurge nettle, a stinging American herb of the Spurge
      family (Jatropha urens).

   Wood nettle, a plant (Laportea Canadensis) which stings
      severely, and is related to the true nettles.
      [1913 Webster]

   Nettle cloth, a kind of thick cotton stuff, japanned, and
      used as a substitute for leather for various purposes.

   Nettle rash (Med.), an eruptive disease resembling the
      effects of whipping with nettles.

   Sea nettle (Zool.), a medusa.
      [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

Rash \Rash\, n. [OF. rasche an eruption, scurf, F. rache; fr.
   (assumed) LL. rasicare to scratch, fr. L. radere, rasum, to
   scrape, scratch, shave. See Rase, and cf. Rascal.] (Med.)
   A fine eruption or efflorescence on the body, with little or
   no elevation.
   [1913 Webster]

   Canker rash. See in the Vocabulary.

   Nettle rash. See Urticaria.

   Rose rash. See Roseola.

   Tooth rash. See Red-gum.
      [1913 Webster]

WordNet (r) 3.0 (2006):

nettle rash
    n 1: an itchy skin eruption characterized by weals with pale
         interiors and well-defined red margins; usually the result
         of an allergic response to insect bites or food or drugs
         [syn: urtication, urticaria, hives, nettle rash]