• September - the ninth month of the year in the Gregorian Calendar.
• Safety and Emergency Procedures - SEP is used in aviation industry by Flight Crew and Cabin Crew to define the procedures used in an emergency.
• The Saturday Evening Post
• SEP IRA - A type of retirement account in the USA.
• Secretaría de Educación Pública - the government department overseeing public education in Mexico.
• System Enhancement Package, an upgrade to the M1A2 Abrams main battle tank.
• Systems Engineering Process - used to develop, implement, and maintain complex systems.
• Sepa - the Heliopolitan manifestation of Osiris as a centipede.
• Single Edge Processor
• Signaling End Point - a term in telecommunications
• Somebody Else's Problem - a fictional cloaking device in The Hitchhiker's Guide to the Galaxy, causing bystanders to disregard whatever was in the field as "somebody else's problem"
• Socialist Equality Party
• Sociedade Esportiva Palmeiras - a Brazilian football team.
• Specialna Enota Policije - a police unit in Slovenia.
• Standard electrode potential
• The Stanford Encyclopedia of Philosophy
• Stanford Exploration Program
• Society for Exact Philosophy
• Solar Electric Propulsion - see ion thruster
• Search engine prominence
• Sep-Se7en, a nickname of Minneapolis rapper Slug
• The Summer Enrichment Program for gifted and talented children in Greeley, Colorado
• Splitterskyddad Enhetsplattform, an armoured vehicle being developed by BAE Systems Hägglunds.
• Société Européenne de Propulsion
• Super Extended Play - a recording mode found on DVD recorders and digital video recorders
• Symantec Endpoint Protection

"Sep." was used on maps to abbreviate Septentrional, the North direction or zone.