The Collaborative International Dictionary of English v.0.48:

Roseola \Ro*se"o*la\, n. [NL., dim. of L. rosa a rose.] (med.)
   A rose-colored efflorescence upon the skin, occurring in
   circumscribed patches of little or no elevation and often
   alternately fading and reviving; also, an acute specific
   disease which is characterized by an eruption of this
   character; -- called also rose rash. -- Ro*se"o*lous, a.
   [1913 Webster]