| I/O | Input/output |
| I/P | Electro-pneumatic (converter) |
| IAASP | International Association of Airports and Seaports |
| IACS | International Association of Classification Societies |
| IAEA | International Atomic Energy Agency |
| IAIN | International Association of Institutes of Navigation |
| IAL | Instrument approach and landing |
| IALA | International Association of Lighthouse Authorities |
| IAOPA | International Aircraft Owners and Pilots Association |
| IAP | Instrument approach procedures |
| Internet access provider |
| IAPH | International Association of Ports and Harbours |
| IAR | Instrument arrival route |
| IAS | Indicated air speed |
| IATA | International Air Transport Association |
| IBC | International bulk carrier |
| IBDS | Integrated biological detection system |
| ibid | Ibidem (in the same place) |
| IBN | Identification beacon |
| IBRD | International Bank for Reconstruction and Development |
| IBS | Integrated bridge system (ship) |
| IC | In charge |
| Integrated circuit |
| Internal combustion |
| ICA | International commodity agreement |
| ICANN | Internet Corporation for Assigned Names and Numbers |
| ICAO | International Civil Aviation Organisation |
| ICAPS | Integrated command anti-submarine warfare prediction system |
| ICB | International Container Bureau |
| International competitive bidding |
| ICBM | Intercontinental ballistic missile |
| ICC | International Chamber of Commerce |
| ICCP | Impressed current cathodic protection |
| ICES | International Council for the Exploitation of the Sea |
| ICHCA | International Cargo Handling Co-ordination Association |
| ICLL | International Convention on Load Lines |
| ICM | Improved conventional munitions |
| ICNIA | Integrated communications, navigation, identification and avionics |
| ICOTAS | International Committee on the Organisation of Traffic at Sea |
| ICP | Inventory control point |
| ICS | International Chamber of Shipping |
| ICSU | International Council for Science |
| ICT | International call for tenders |
| ICV | Infantry combat vehicle |
| ID | Identification |
| Import duty |
| IDAS | Integrated defence aid system |
| IDC | Insulation displacement connector |
| International digital communications |
| IDE | Integrated device electronics |
| Integrated drive electronics |
| IDF | Interceptor day fighter |
| IDR | Instrument departure route |
| IDS | Intruder detection system |
| IE | Index error (nav) |
| ie | id est (that is to say) |
| IEC | International Electrotechnical Commission |
| IED | Improvised explosive device |
| IEDD | Improvised explosive device disposal |
| IEE | Institution of Electrical Engineers |
| IERE | Institution of Electronic & Radio Engineers (now part of IEE) |
| IERS | International earth rotation service |
| IETF | Internet Engineering Task Force |
| IETM | Integrated electronic technical manuals |
| IETP | Integrated electronic technical publication |
| IF | Intermediate frequency |
| IFA | International Federation of Airworthiness |
| IFALPA | International Federation of Airline Pilots´ Associations |
| IFATCA | International Federation of Air Traffic Controllers´ Associations |
| IFB | Invitation for bid |
| IFC | International Finance Corporation |
| IFCS | Improved fire control system |
| IFF | Identification, friend-or-foe |
| IFL | International frequency list |
| IFM | Instantaneous frequency measurement |
| IFO | Intermediate fuel oil |
| IFR | Instrument flight rules |
| IFRB | International Frequency Registration Board |
| IFRS | International Financial Reporting Standard |
| IFV | Infantry fighting vehicle |
| IFZ | Inshore fisheries zone |
| IG | Image generator |
| IGBT | Inverse gate bipolar transistor |
| IGC | International gas carrier |
| IGE | In ground effect |
| IGES | Initial graphics exchange specification (computer) |
| IGFET | Insulated gate field effect transistor |
| IGOS | Integrated global observing system (of CEOS) |
| IGOSS | Integrated Global Ocean Service System |
| IGS | Inert gas system |
| Inertial guidance system |
| IHADSS | Integrated helmet and display sight system |
| IHO | International Hydrographic Organisation |
| II | Image intensifier |
| IIP | International ice patrol |
| IIT | Image intensifier tube |
| IKBS | Intelligent knowledge-based system (computer) |
| ILGS | Instrument landing guidance system |
| ILS | Instrument landing system |
| Integrated logistic support |
| ILSM | Integrated logistic support manager |
| ILSN | Integrated logistic support number |
| ILSP | Integrated logistic support plan |
| IM | Inner marker |
| Instant messaging |
| Interceptor missile |
| IMC | Instrument meteorological conditions |
| IMCO | Intergovernmental maritime consultative organisation |
| IMDEX | International marine traffic data exchange |
| IMDG | International maritime dangerous goods (code) |
| IMF | International Monetary Fund |
| IMIF | International Maritime Industry Forum |
| IMM | Integrated materials management |
| IMMLC | Improved medium-mobility load carrier (truck) |
| IMO | International Maritime Organization |
| IMP | Initial military programme |
| Internal message procedure |
| IMS | Ion mobility spectrometry |
| IN | Inertial navigation |
| in | Inch (0.025 m) |
| INAS | Inertial navigation and attack system |
| INF | Intermediate-range nuclear forces |
| inf | Infantry |
| Inmarsat | International Maritime Satellite Organisation |
| INS | Inertial navigation system |
| int | Intelligence |
| intl | International |
| intsum | Intelligence summary |
| IO | Intelligence officer |
| IOC | Initial operational capability |
| Intergovernmental Oceanographic Commission (of UN) |
| IOCS | Input/output control system (computer) |
| IODE | International Ocean Data and Information Exchange |
| IOP | Initial public offering (shares) |
| IOT | Input/output terminal |
| IOTTSG | International Oil Tanker and Terminal Safety Group |
| IP | Initial point |
| Initial provisioning |
| Internet protocol |
| IPAR | Improved pulse acquisition radar |
| IPARS | International passenger reservation system |
| IPC | Illustrated parts catalogue |
| Integrated pollution control |
| IPCC | Intergovernmental panel on climate change (UN) |
| IPL | Initial provisioning list |
| IPO | Initial public offering |
| IPP | Internet printing protocol |
| IPR | Intellectual property rights |
| IPSS | Initial provisioning support system |
| IPT | Integrated project team |
| IPV | Inshore patrol vessel |
| IR | Infra-red |
| Instrument rating (pilot´s) |
| IRBM | Intermediate range ballistic missile |
| IRCM | Infra-red counter-measures |
| IRFC | Infra-red false colour |
| IRLS | Infra-red linescan system |
| IRM | Inspection, repair and maintenance |
| IRQ | Interrupt request |
| IRS | Inertial reference system (nav) |
| Initial readiness site |
| Internal radar service |
| IRU | Inertial reference unit (nav) |
| IRVR | Instrumented runway visual range |
| IS | Internal security |
| ISA | Industry standard architecture (computers) |
| International standard atmosphere (101 325 Pa or 760 mm Hg) |
| ISBN | International standard book number |
| ISD | In-service date |
| ISDN | Integrated services digital network |
| ISF | International Shipping Federation |
| ISGOTT | International safety guide for oil tankers and terminals |
| ISIC | International standard industrial classification |
| ISID | International ship identification database |
| ISIS | Integrated ship instrumentation system |
| Integrated strike and interceptor system |
| Item of supply information system |
| ISM | Industrial, scientific and medical (radio band) |
| International safety management (code) |
| ISMO | Information system management office |
| ISO | Infra-red space observatory |
| International Standards Organisation |
| ISO14000 | Environmental management standard certified by ISO |
| ISO17799 | Security standard certified by ISO |
| ISO9000 | Quality standard certified by ISO |
| ISP | Integrated support plan |
| Internet service provider |
| ISPS | International shipping and port security |
| ISS | International space station |
| ISSP | Integrated supply support procedures |
| ISTAR | Intelligence, surveillance, target acquisition and reconnaissance |
| IT | Information technology |
| ITA | International telegraph alphabet |
| ITDM | Intelligent time-division multiplexing (telegraphy) |
| ITEA | Integrated test, evaluation and acceptance |
| ITER | International Thermonuclear Experimental Reactor |
| ITIC | International tsunami information centre |
| ITP | Intercept terminal point (nav) |
| ITSEC | Information technology security evaluation and certification scheme |
| ITT | Invitation to tender |
| ITU | International Telecommunications Union (formerly CCITT) |
| ITZ | Inshore traffic zone |
| IUKADGE | Improved UK air defence ground environment |
| IVSN | Initial voice-switched network |
| IW | Individual weapon |
| Inland waters |
| IWS | Interoperable waveform standard (communications) |
| IWT | Inland waterways transport |
| International Water Tribunal |
