In movable-do solfège, ソ is the fifth note of the major scale. Used in music education and sight-singing.
from Italian
This piece starts on so.
In the fixed-do system, ソ always refers to the note G, regardless of key. Common in classical music contexts.
Please hit the G note accurately.
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 'so', the fifth note of the solfège scale. The fixed-do meaning comes from the same system, where each solfège syllable is permanently assigned to a specific pitch.