Sei or SEI may refer to:

• Sei (album), by Brazilian singer and composer Nando Reis
  • "Sei", the title-song and first single from that album
• Sei (film), a 2018 Tamil thriller film

• 7-Eleven, a popular multinational chain of gas stations and convenience stores
• Safety Equipment Institute, a private, non-profit organization established to test safety and protective products
• Scottish Episcopal Institute, the theological college of the Scottish Episcopal Church
• Script Encoding Initiative, a department at UC Berkeley supporting proposals for minor and historic scripts in the Unicode Standard.
• Scuba Educators International, a non-profit diver training organization
• SEI Investments Company, a financial services company headquartered in the United States
• Slough Estates International, the former name for Segro
• Smith Enterprise, a firearm and accessory manufacturing facility based in Tempe, Arizona that is known for making flash suppressors, muzzle brakes, sound suppressors, M14 rifles and accessories for M14 rifles.
• Software Engineering Institute, a federally funded research and development center at Carnegie Mellon University
• SpaceWorks Enterprises, a small aerospace engineering company in the United States
• Stockholm Environment Institute, a non-profit institute that specializes in sustainable development and environmental issues
• Sustainable Endowments Institute, a special project of Rockefeller Philanthropy Advisors, Inc, the Sustainable Endowments Institute (SEI) has pioneered research, education and outreach to advance resilient institutional responses to the climate crisis.
• Sumitomo Electric Industries, a manufacturer in automotive, information and communications, electronics, environment and energy, and industrial materials
• SUNY Eye Institute, a medical and basic sciences research institute in New York

Given name

• Shō Sei (1497–1555), king of the Ryukyu Kingdom from 1526 to 1555
• Shō Sei (r. 1803), king of the Ryukyu Kingdom in 1803
• Sei Fujii (1882–1954), Japanese male immigrant rights activist in the United States
• Sei Hatsuno (born 1976), Japanese male writer of mystery and thriller novels
• Sei Ashina (born 1983), Japanese actress
• Sei Kawahara (born 1995), Japanese male figure skater
• A member of girl group Weki Meki

Surname

• Indrek Sei (born 1972), Estonian freestyle swimmer

• Sei whale, a baleen whale
• Se'i, traditional bacon from Timor, Indonesia
• Seebeck effect imaging, uses a laser to generate thermal gradients in conductors in order to locate electrically floating conductors
• Secondary electron image, a type of imaging in scanning electron microscopes to view sample topography
• Seri language, spoken in two Mexican villages
• Service endpoint interface, a term used in Java Platform, Enterprise Edition when exposing Enterprise JavaBean as web service
• Social-Emotional Intelligence (see Emotional Intelligence)
• Solid-electrolyte interphase (see Lithium-ion battery)
• Space Exploration Initiative, a plan envisioned by former U.S. President George H.W. Bush with crewed Moon and Mars missions
• Spanish East Indies, a former Spanish colony comprising the Philippines and most of Micronesia

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