The Collaborative International Dictionary of English v.0.48:

Measles \Mea"sles\, n.; pl. in form, but used as singular in
   senses 1, 2, & 3. [D. mazelen; akin to G. masern, pl., and E.
   mazer, and orig. meaning, little spots. See Mazer.]
   [1913 Webster]
   1. (Med.) A contagious viral febrile disorder commencing with
      catarrhal symptoms, and marked by the appearance on the
      third day of an eruption of distinct red circular spots,
      which coalesce in a crescentic form, are slightly raised
      above the surface, and after the fourth day of the
      eruption gradually decline; rubeola. It is a common
      childhood disease.
      [1913 Webster +PJC]

            Measles commences with the ordinary symptoms of
            fever.                                --Am. Cyc.
      [1913 Webster]

   2. (Veter. Med.) A disease of cattle and swine in which the
      flesh is filled with the embryos of different varieties of
      the tapeworm.
      [1913 Webster]

   3. A disease of trees. [Obs.]
      [1913 Webster]

   4. pl. (Zool.) The larvae of any tapeworm (Taenia) in the
      cysticerus stage, when contained in meat. Called also
      bladder worms.
      [1913 Webster]

   German measles A mild contagious viral disease, which may
      cause birth defects if contracted by a pregnant woman
      during early pregnancy; also called rubella.
      [PJC]

The Collaborative International Dictionary of English v.0.48:

rubella \ru*bel"la\, n. [NL., fr. L. rubellus reddish.] (Med.)
   An acute but mild viral infection characterized by a dusky
   red cutaneous eruption resembling that of measles, but
   attended by only mild respiratory problems or fever; --
   called also German measles. The infective virus is called
   Rubella virus, or Rubivirus. If contracted by a woman
   during the first several months of pregnancy, rubella may
   cause serious abnormalities in the fetus.
   [1913 Webster +PJC]

WordNet (r) 3.0 (2006):

rubella
    n 1: a contagious viral disease that is a milder form of measles
         lasting three or four days; can be damaging to a fetus
         during the first trimester [syn: German measles,
         rubella, three-day measles, epidemic roseola]