SN

SN stands for

EU jargon

 * Speaking Note

IT

 * Surname, an attribute of the LDAP protocol family
 * screenname used in instant messaging

Social media

 * Social Network

Other

 * Serial Number
 * SpaceX's Starship Serial Number 10 (SN10)
 * sine nomine, Latin for "without a name", used in bibliographical catalogs to indicate publisher, etc is unknown
 * "side note" used in messages like "P.S."