Definition and background:
The raised seventh note ( leading tone) in the C major scale. This pitch has no solfeggio representation because the actual pitch is the same as the tonic or eighth note ( C) in the fixed do system. However, the note name B-sharp is necessary in certain instances such as the key of C-sharp major where B-sharp is the seventh note ( leading tone) or A-sharp minor where B-sharp is the second note ( supertonic). The frequencies of the audible pitches of B-sharp are notated below. See also [Eng.] B-Sharp; [Fr.] Si diese; [Ger.] His; [It.] Si diesis; [Sp.] Si sostenido.
For more information on the music term "B-sharp" check out these other resources:
Wikipedia - Glossary of Musical Terminology
Virginia Tech Multimedia Music Dictionary
ORB -- Medieval Music Glossary