PC - Player Character, the fictional person played by the simmer.
NPC - Non-Player Character, a fictional person used by all the simmers.
IC - In Character, something a PC does.
OOC - Out Of Character, used by the simmer to clarify or question something. (i.e. how do you spell Ferengi?)
NRPG - Non-Role Playing Game, used by a simmer for non-sim related information or questions. (i.e. did you see last night's episode?)
RL - Real Life, used to describe something in the simmers real life. (i.e. I'm getting married!)
CO - Commanding Officer, the highest ranking officer of the ship/station.
GM - Game Master, the person who runs & manages the sim, usually the CO.
XO - Executive Officer, the first officer, the assistant CO, second in command.
CTO - Chief Tactical Officer, head of the Tactical Department.
CMO - Chief Medical Officer, head of the Medical Department.
CFO - Chief Flight Control Officer, head of the Flight Control Department. (Conn., Helm)
COO - Chief Operations Management Officer, head of the Operations Management Department. (Ops)
CSO - Chief Security Officer, head of the Security Department.
CEO - Chief Engineering Officer, head of the Engineering Department.
ACEO - Assistant Chief Engineering Officer, assistant head of the Engineering Department