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

UEGO = Upstream Exhaust Gas Oxygen sensor (UHEGO, HO2, HO2S, Lambda Sensor, O2S)
UHEGO = Upstream Heated Exhaust Gas Oxygen sensor (HEGO, HO2, HO2S, O2S, UEGO)
UHP = Ultra-High Performance, a manufacturer’s classification for tires/tyres
U-joint = Universal joint (UJ)
UJ = Universal Joint (U-joint)
Underfelt = Thick natural jute with rubber coating used as Jaguar carpet padding and soundproofing
Underscuttle = Knee bolster, trim panel, finishing panel, etc.
UNF = Unified National Fine Thread (see UTS)
Unibody = Monocoque
Unmetered = Not measured by the EMS
Unmetered Air = Air drawn into Intake Manifold but not measured by the MAFS (e.g. vacuum leak)
Unmetered Fuel = Fuel drawn into Intake Manifold but not measured by the ECM (e.g. leaking FPR)
Upstream = Earlier/preceding in a circuit/system, as in Upstream O2S (Pre Cat) (see Downstream)
U.T. = United Technologies (w.s.)
UTS = Unified Thread Standard, ASME/ANSI standard for threads used in the U.S.A. & Canada
UV = Ultraviolet, EMR with wavelengths shorter than visible light, longer than X-rays (see EMR, IR)