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

I = Symbol for Current in electronics equations
I6 = Inline six-cylinder engine, Straight Six engine
IA = Intake Air
IAC = Idle Air Control or Idle Air Control Valve (IACV), see also ISC, ISCM, ISSM
IACV = Idle Air Control Valve (IAC), see also ISC, ISCM, ISSM
IAT = Intake Air Temperature, Intake Air Temperature Sensor (IATS, ACT, CTS, MAT, MCT, VAT)
IATS = Intake Air Temperature Sensor (IAT, ACT, CTS, MAT, MCT, VAT)
IBWM = Int’l Bureau of Weights and Measures, Bureau International des Poids et Mesures (BIPM)
IC = Instrument Cluster (IP, IPK, ICCM), Internal Combustion, Industry/Industrie Canada, integrated circuit
ICCM = Instrument Control Center Module (IC, IP, IPK)
ICE = In-Car Entertainment System
ICM = Ignition Control Module (ICU)
ICU = Ignition Control Unit (ICM)
ID or I.D. = Identification, or Inside Diameter (see OD)
Idle = Tick Over, or the engine speed at idle
IDM = Injector Driver Module
IDS = Integrated Diagnostic Software, Ford/JLR computer-based diagnostics (See PDU, SDD, WDS)
i.e. = id est (Latin): that is, that is to say
IFS = Independent Front Suspension
IG = Ignition
IGN = Ignition
IIABDFI = “If It Ain’t Broken Don’t Fix It” (IIIBDFI)
IIRC = “If I recall correctly”
IIIBDFI = “If It Isn’t Broken Don’t Fix It” (IIABDFI)
ILSAC = Int’l Lubricant Standardization & Approval Committee, motor oil standards, see ACEA, API
Imax = Current at which a fuse melts; Breaking Capacity, Interrupting Rating, Short-Circuit Rating
Impeller = Bladed rotor for transmitting motion to a fluid, as in a water pump, torque converter, etc.
Imperial = British Imperial measurement units: Inch, Foot, Yard, Mile, Quart, Gallon, Pound (SAE)
IMT = Intake Manifold Tuning Valve (see VIS)
IMT = Valve Intake Manifold Tuning Valve; Valve 1 = Top, Valve 2 = Bottom
IMHO = In my humble opinion
IMO = In my opinion
IMT = Intake Manifold Tuning Valve, LH or RH (LHIMT, RHIMT)
In Good Nick = In good condition
Indicators = Direction Indicators, Turn Signals, Signals (DI)
inHg = Inches of Mercury, unit of pressure, measure of engine manifold vacuum, 1 inHg = 3386.389 Pa
Inline Six = Straight Six engine (I6)
Interactive Voice = Jaguar voice-recognition control system
Interstate = Autobahn, Freeway, Highway, Main Road, Motorway, Parkway
Interrupting Rating = Current at which a fuse melts; Imax, Breaking Capacity, Short-Circuit Rating
Int’l = International
I/O = Input/Output
IP = Injection Pressure, Injector Pressure Sensor, Fuel Injection Pressure Sensor
IP or I/P = Instrument Pack or Panel (IC, ICCM, IPK)
IPC = Ignition Primary Control (see ISC)
IPD = Injector Pulse Duration (On Time, Pulsewidth, Pulse Width)
IPDA = Definition not yet known, probably diesel-related; from p. 208 of X350 Owner’s manual
IPR = Ignition Positive Relay
IPS = Injection Pressure Sensor, Fuel Injection Pressure Sensor
IPK = Instrument Pack (IC, IP)
IR = Infrared, radiant energy/EMR with wavelengths longer than visible light (see EMR, UV)
IRS = Independent Rear Suspension, Jaguar’s, designed by Bob Knight, is one of the world’s best
IS = Inertia Switch, Intrusion Sensor or Intrusion Sensor Module (ISM)
ISC = Idle Speed Control (see also IAC, IACV, ISCM, ISSM)
ISC = Ignition Secondary Control (see IPC)
ISCM = Idle Speed Control Motor (ISSM), see also ISC, IAC, IACV
ISM = Intrusion Sensor Module (IS)
ISR = Idle Speed Reference
ISSM = Idle Speed Stepper Motor (ISCM), see also IAC, IACV, ISC, ISCM)
ISO = International Standards Organization, establishes various industrial standards
ISO = Serial Data Link Network (ISO 9141)
ISO Relay = Relay conforming to ISO automotive relay standards (ISO, Mini ISO & Micro ISO sizes)
ISO 9001 = Fundamental quality management systems & standards for automotive makers & suppliers
ISOFIX = Child safety seat restraint system
ISO 15765 CAN = Bosch protocol required on all U.S. cars from 2008 (see CAN, CAN ISO 15765)
ISO/TS 16949 = Quality assurance standard for OEM parts suppliers that superseded QS9000
ISO 9141 = Serial Data Link Network
ISU = International System of Units (SI)
ITA = Ignition Timing Advance (measured in degrees)
IVT = In-Vehicle Temperature, In-Vehicle Temperature Sensor (see IVTH, IVTS, IVTHS)
IVTH = In-Vehicle Temperature and Humidity, In-Vehicle Temperature and Humidity Sensor
IVTHS = In-Vehicle Temperature and Humidity Sensor (IVT, IVTH, IVTS)
IVTS = In-Vehicle Temperature Sensor (see IVT, IVTH, IVTHS)