ti; si
In movable-do solfège, the seventh note of a major scale. In fixed-do systems, this corresponds to B.
from Italian
The シ in do-re-mi-fa-sol-la-ti is the Italian note name.
In the fixed-do solfège system, シ always represents the note B, regardless of key.
In fixed-do, シ always refers to the note B.
ソ is the fifth note of the scale (sol/G), while シ is the seventh note (ti/B), and both are single-katakana solfège syllables that can be mixed up.
Borrowed from Italian 'si', the seventh note of the solfège scale.