SY

Short definition from Wiktionary

Noun

1. 1
   Initialism of steam yacht: a steam-powered yacht.
2. 2
   Initialism of sailing yacht: a wind-powered yacht.
3. 3
   Initialism of school year.
4. 4
   Abbreviation of Seyfert galaxy, a type of AGN class of active galaxy.

Proper noun

1. 1
   Alternative form of Si.
2. 2
   A Chinese Filipino surname from Spanish