The Collaborative International Dictionary of English v.0.48:

Semibreve \Sem"i*breve`\, n. [Pref. semi- + breve: cf. F.
   semi-breve, It. semibreve.] [Formerly written semibref.]
   (Mus.)
   A note of half the time or duration of the breve; -- now
   usually called a whole note. It is the longest note in
   general use.
   [1913 Webster]