D defs.my
Entry 1 sense Webster, 1913

Si

/sû/ · IPA /siː/
01 A syllable applied, in solmization, to the note B; more recently, to the seventh tone of any major diatonic scale. It was added to Guido's …
  1. 1.
    A syllable applied, in solmization, to the note B; more recently, to the seventh tone of any major diatonic scale. It was added to Guido's scale by Le Maire about the end of the 17th century.(Mus.)