USS means United Space Ship, as a way to identify Federation starships. The original USS means United States Ship, & is used by the US Navy.
There are also some variants. In the Mirror Universe, the Terran starships use ISS, that means Imperial Star Ship. In some non-canon novels, the timeship Aeon is identified as UTS Aeon, UTS means United Time Ship. In another non-canon alternate realities novels, there's FSS (Federation Star Ship), USV (United Star Vessel), and even US (United Ship).
I thought it was United Star Ship, which would make more sense - they never call them spaceships except to people who are from the past (if I remember correctly). Also, this would mean the SS Tsiolkovsky's 'SS' referred simply to 'Star Ship'.
Kitface 11:08, June 14, 2011 (UTC)