SSI has many meanings, depending on the topic:

Engineering

• Soil structure interaction, an interaction between soil and structure during earthqaukes

Economics

• Standard Settlement Instructions, payment processing and settlement information about financial institutions
• Strategic Sustainable Investing, an investment strategy that recognizes financial value in transitional leadership towards sustainability
• Supplemental Security Income, a United States federal government program

Computing

• Server Side Includes, a server-side scripting language used on the web
• Server System Infrastructure, design specifications for common server hardware elements
• Single Sign In, a specialized form of software authentication
• Single-system image, a cluster dedicated operating system
• Small Scale Integration, electronic chips
• Synchronous Serial Interface, electronics

Medicine

• Sliding-Scale Insulin, a method of insulin administration
• Solid State Intelligence, a malevolent entity described by John C. Lilly
• Stress-Strain Index, a measure of bone strength
• Surgical Site Infection, this term is preferred over "Surgical Wound Infection"

Military and Police

• Selective Service Initiative, a form of military conscription in the United States
• Senior Station Inspector, a rank in the Singapore Police Force
• Serviciul Secret de Informa?ii, the Romanian intelligence service in the interwar period (until November 1940)
• Serviciul Special de Informa?ii, the Romanian intelligence service between November 1940 and September 1944
• Shoulder Sleeve Insignia (US Army), formation insignia used by the United States Army
• Staff Sergeant Instructor, an appointment in the British Army
• Subsecretaria de Inteligência, the Brazilian intelligence agency until 1995

Organizations

• Samsung Semiconductor Inc., a Korean electronics company
• Scuba Schools International, a diver training organization
• Southern Software, Inc. (SSi), a type foundry
• Space Services Inc., a team of companies investigating new commercial opportunities in space
• Space Studies Institute, an American non-profit organisation
• Spatial Sciences Institute, a professional body in the Asia-Pacific region
• Statens Serum Institut, the Danish State Serum Institute
• Strategic Simulations, Inc., a software company producing simulation games
• Strategic Studies Institute, the United States Army War College institute for geostrategic and national security research and analysis
• Sustainable Sciences Institute, a San Francisco-based organisation building local capacity in developing countries
• Statens strålskyddsinstitut (Swedish Radiation Protection Authority), a Stockholm-based regulatory and research government agency.
• Swedish Society for Interlingua, a society that encourages the active use of Interlingua in Sweden.

Transportation

• Sensitive Security Information, a designation used by the United States Transportation Security Administration
• Solid State Interlocking is the brand name of the first generation processor-based interlocking developed in the 1980s by British Rail, GEC-General Signal and Westinghouse Signals Ltd in the UK.

Honorifics

• Star of the Solomon Islands, the most senior award made by the government of the Solomon Islands

Market Research

• The SSI model, stands for Subject Matter Expertise, Site visits, Industry Consultations

Popular Culture

• Sonic Solutions Incorporated, an electronic music group

de:SSI eo:SSI fr:SSI it:SSI he:SSI nl:SSI ja:SSI pl:SSI sv:SSI zh:SSI