This glossary was compiled by Don B at jaguarforums.com and is reproduced here
with his permission

R = Reverse; or Resistance in electronic equations
(R+M/2) = Fuel octane or performance rating derived from average of RON and MON (AKI)
R-12 = DuPont Freon CFC refrigerant used in auto A/C systems, phased out after Montreal Protocol
R-134a = Refrigerant 134a, tetrafluoroethane (C2H4F4), HFC refrigerant that replaced R-12
R-152a = Refrigerant 152a, difluoroethane (C2H4F2), may replace R-134a in autos (see HFO-1234yf)
RAM = Random Access Memory
RATC = Rear Automatic Temperature Control Module (not to be confused with RCCM)
RCM = Restraints Control Module or Resistance Deployment Control Module
RCCM = Remote Climate Control Module (not to be confused with RATC)
RCCP = Rear Climate Control Panel
RLCM = Rear Lighting Control Module
RCMISO = Resistance Deployment Control Module ISO bus
RCMSCP = Resistance Deployment Control Module Secondary Control Processor
RDS = Radio Data System(s)
RECM = Rear Electronic Control Module (REM)
RECP = Rear Entertainment Control Panel
Redesign = Major design and engineering changes to an auto model (see also Facelift)
Reluctor = Sensor Ring, sensor rotor, trigger wheel
REM = Rear Electronic Module (RECM)
Resonator, Helmholtz = Tuned chamber designed to reduce noise, especially in exhausts or air intakes
Restricted Performance = Limited transmission and/or engine function due to a fault (LHM, RP)
RETM = Rear Entertainment Control Module, Rear Seat Entertainment Control Module
Rev = Reverse or Revolution
Rev Counter = Revolution Counter, Tachometer, Tach or TACH
Revolution Counter = Rev Counter, Tachometer, Tach or TACH
Rexine = Thin early forerunner of vinyl used on XK140 and earlier interiors (see vynide)
RF = Radio Frequency
RFI = Radio Frequency Interference (see EMR, EMI)
RH = Right Hand, Right Hand Side, typically referenced from the driver’s seat facing forward
RHD = Right Hand Drive
Rheostat = A two-terminal variable resistor used to control current, as in a light dimmer
Rheostat Switch = A combination rheostat and On/Off switch
RHIMT = Right-Hand Intake Manifold Tuning Valve, (IMT, LHIMT)
RHLCM = Right Hand Level Control Module (ADHLS R)
RHS = Right Hand Side, typically referenced from the driver’s seat facing forward
Rich = AFR with too little air and/or too much fuel, Lambda (λ) < 1 (see Lean)
Rimbellisher = Decorative trim ring for outer rim of wheel, often chrome plated
Ring Gear = Crown Wheel (in differential), or Starter Gear on flywheel or flex plate
Ring Terminal = Eyelet Terminal
Ring Spanner = Box End Wrench or Box Wrench
RKE = Remote Keyless Entry
RME = Rapeseed Methyl Ester, a biodiesel fuel produced from low erucic acid varieties of canola seed
RMM = Rear Memory Module
Roadster = Non-hardtop coupe, may or may not have a convertible top or hood
Rocker Cover = Protective cap on overhead-valve engine, Valve Cover, see Cam Cover
Rocker Panel = Sill
ROM = Read-Only Memory
RON = Research Octane Number, a fuel performance rating system; see MON,
(R+M/2), AKI
Roots = A Roots-type supercharger or blower, positive displacement lobe pump
Rotor = Brake disc or distributor rotor
ROW = Rest of World, Rest of the World Specification
RP = Restricted Performance, permits driving while reducing risk of engine or trans damage (LHM)
RPaid = Rear Parking Aid
RPDB = Rear Power Distribution Box, Rear Power Distribution Fuse Box
rpm or RPM = Revolutions Per Minute
RS = Road Speed, typically expressed in mph o km/h (VS, VRS)
RSE = Rear Seat Entertainment
RSIS = Rear Seat Infotainment System
RSM = Rear Seat Module, Rear Seat Control Module, Rear Head Restraint Module
RTFM = Read the f**king manual
RTM = Read the manual
Rug = Footwell rug, overmat or carpet
RWD = Rear Wheel Drive (see FWD)
RWHP = Rear Wheel Horsepower