Glossary

The purpose of this document is, mostly, to define the stuff that you can't super easily get off Wikipedia. There's still a decent bit of that, but this is designed to be relatively specific to RO. Stock KSP terms are also not included.

Rocket Design/VAB:

COL: Center of lift (Can be displayed in the VAB)
COM: Center of mass (Can be displayed in the VAB)
MLI: Multi-layer insulation
PAW: Part action window. It's the menu that pops up when you right-click a part.
SLT: Sea-level TWR
TWR: Thrust-to-weight ratio

Spaceflight:

Apogee: The apoapsis of an orbit around the Earth. Perigee is the opposite.
Aposelene: The apoapsis of an orbit around the Moon. Periselene is the opposite.
LES: Launch Escape System
SM: Service module
TVC: Thrust vector control. A method of steering a rocket by changing which way the thrust is pointed. The primary methods are either by gimbaling the whole engine to point a different direction, or by using thrust veins that dip into the exhaust.
Kerolox: Using kerosene and liquid oxygen as propellants.
Ethalox/alcolox: Using ethanol and liquid oxygen as propellants.
Hydrolox: Using liquid hydrogen and liquid oxygen as propellants.
Hypergolic: Propellants that will spontaneously combust if mixed.
Ullage: The empty space in a tank. In microgravity, it's hard to keep the gas opposite the pipes leading to the engines, which can prevent engines from lighting. Methods of dealing with this problem may also use this, for example "ullage motors".

Space Agencies:

ESA: European Space Agency
ISA: Iranian Space Agency
ISA: Israel Space Agency
ISRO: Indian Space Research Organization
JAXA: Japan Aerospace Exploration Agency
KARI: Korea Aerospace Research Institute (South Korean)
NASA: National Aeronautics and Space Administration. (U.S.)
Roscosmos: Russian space agency

Mods:

AA: Atmosphere Autopilot, the best way to fly planes in RO.
Agathorn: Creator of the mod Test Flight.
AJE: Advanced Jet Engine
DU: Data Units. Part of Test Flight, these symbolize how the knowledge you gain from using engines increases reliability of that engine.
EEX: Editor Extensions reduX
EVE: Environmental Visual Enhancements
FAR: Ferram Aerospace Research, the aero model used by RO.
KAC: Kerbal Alarm Clock
MLP: Modular Launch Pads
MM: Module Manager, a modding tool that can make it easy to change the properties of a part in another mod.
NK: Nathan Kell, a developer of RO mods.
PLC: Programs and Launch Complexes, a (currently) experimental branch of RP-1. See the Discord channel for more info.
PVG: Primer Vector Guidance, a mode for MechJeb's ascent guidance that will allow you to reach orbit in RO with autopilot. Read the articles in the side bar to figure out how to use it.
RA: Real Antennas
RF: Real Fuels
RiS: Race into space, a RP-1 competition happening over on Discord.
RN: Raider Nick, a developer of RO mods.
RO: Realism Overhaul
ROE: Realism Overhaul Engines
RSS: Real Solar System
RSSVE: Real Solar System Visual Enhancements
TF: Test Flight
TL: Test Light
TU: Textures Unlimited
TUFX: A visual mod

Contracts:

FO: First Orbit
FSO/FSS: First Scientific Orbit/First Scientific Satellite, or sometimes Solar instead of Scientific.