A user in Second Life is typically called a Resident (or Resi). This term is used by Linden Lab and may be meant to give users a feeling of "belonging" and ownership of the virtual world. "Resident" is also used throughout most of SL's UI in place of "user".
Origin of the term 'Resident'
It was pretty early in the development of Second Life. One of my jobs was to come up with the different words we wanted to use, including the name! The naming discussions were usually between me, Philip, Hunter and Peter (both of whom aren't here any more) with feedback from everyone else.
When it came to what to call the people in the world, we knew we didn't want to call them 'users', although that would be the most typical thing for software. However, the word 'users' doesn't do a very good job of describing the two-way nature of Second Life, where the people involved are providing content and contributing to the experience.
We also thought about 'members' (boring!), 'citizens' (too political!), and 'players' (too game-y).
'Residents', however, seems most descriptive of people who have a stake in the world and how it grows.
-- Robin LindenP.S. Because it's a title, we've chosen to capitalize Resident, when referring to the inhabitants of Second Life. Sometimes this term has been abbreviated to "Resi's".