OXFORD DICTIONARY
si, abbr.
1 (Order of the) Star of India.
2 the international system of units of measurement (F Syst{egrave}me International).
si, symb. Chem. the element silicon.
si, n. Mus. = TE.
F f. It., perh. f. the initials of Sancte Iohannes: see GAMUT