NI
- Ni – (s) Nickel
- NI
- (s) Nicaragua (ISO 3166 digram)
- Nigeria (FIPS 10-4 country code)
- NIAAA – (i) U.S. National Institute on Alcohol Abuse and Alcoholism
- NIAID – (a) National Institute of Allergies and Infectious Disease
- NIAG – (a) NATO Industrial Advisory Group
- NIAMD – (i) National Institute of Arthritis and Musculoskeletal Diseases
- NIB – (i) New In Box (Internet auction/trading listings)
- NIBP – (i) Non-invasive blood pressure monitor
- NIC
- (a) Network Interface Card
- (s) Nicaragua (ISO 3166 trigram)
- NicA – (i) Nicotine Anonymous
- NICAP – (a) National Investigations Committee on Aerial Phenomena
- NICE – (p) UK. National Institute for Health and Clinical Excellence
- NICHD – National Institute of Child Health and Human Development
- NICP – (i) National Inventory Control Point
- NIDDK – (i) National Institute of Diabetes and Digestive and Kidney Diseases
- NIDRR – (i) National Institute on Disability Research and Rehabilitation
- NIEHS – (i) National Institute of Environmental Health Sciences
- NIG – (s) Niger (IOC and FIFA trigram, but not ISO 3166)
- NIH
- (i) National Institutes of Health
- Not Invented Here
- NIIN – (i) National Item Identification Number
- NIMA – (a) National Imagery and Mapping Agency (became NGA)
- NIMBY – (a) Not In My Back Yard (See NOME)
- NIMG – (i) Not In My Generation
- NiMH – (p) nickel metal hydride ("Ni" is the chemical symbol for nickel)
- NIMH
- (i) National Institute of Medical Herbalists
- National Institute of Mental Health
- NIN – (i) Nine Inch Nails (band)
- NIO – (s) Nicaraguayan cordoba oro (ISO 4217 currency code)
- NIPR – (i) National Institute of Polar Research (Japan)
- NIR – (s) Northern Ireland (FIFA trigram; not eligible for an ISO 3166 or IOC trigram)
- NISAC – (a) U.S. National Infrastructure Simulation and Analysis Center
- NISCAP – (a) NSA Information System Certification and Accreditation Program
- NISER – (i) National Institute of Science Education and Research
- NIST – (a/i) National Institute of Standards and Technology
- NIT
- (i) National Invitation Tournament (U.S. college basketball)
- (i) National Institutes of Technology (India)
- NITFS – (i) National Imagery Transmission Format Standard
- NIU – (s) Niue (ISO 3166 trigram)
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