NIO is an acronym with several meanings:

• National Institute Of Oceanography, India, a constituent laboratory of CSIR - the Council of Scientific and Industrial Research, a research body of the Indian government
• National Institute of Ophthalmology
• Neurotechnology Industry Organization
• New I/O (usually "NIO"), a collection of Java programming language APIs introduced with the J2SE 1.4 release of Java by Sun Microsystems
• Nicaraguan córdoba (ISO 4217 code: NIO), the national currency of Nicaragua
• Noise-induced oscillation
• Northern Ireland Office, an arm of the United Kingdom government, responsible for Northern Ireland affairs

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