Source : Webster's Revised Unabridged Dictionary (1913)

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.

Source : WordNet®

semibreve
     n : a musical note having the longest time value (equal to four
         beats in common time) [syn: {whole note}]