| Abbreviation | Definition |
|---|---|
| SI | SAS Institute |
| SIA | Safety Institute of Australia |
| SIAC | Security Industry Alarm Coalition |
| SIAC | Spatial Information and Analysis Consortium |
| SIAS | Sussex Industrial Archaeology Society |
| SICA | Shore Institute of the Contemporary Arts |
| SICA | Stone Institute for Challenged Adolescents |
| SIDA | Swedish International Development Agency |
| SIDSA | Sudden Infant Death Syndrome Alliance |
| SIECUS | Sexuality Information and Education Council of the United States |
| SIETAR | Society for Intercultural Education, Training, And Research |
| SIFE | Students in Free Enterprise |
| SIFT | Student Initiative Fund for Technology |
| SIG | Special Interest Group |
| SII | Sensory Integration International |
| SII | Stellar International Institute |
| SIK | Soroptimist International of Kona |
| SIL | Societas Internationalis Limnologiae |
| SIMM | Singapore Institute of Materials Management |
| SION | Stop Islamization of Nations |