- Initialism of Central European Initiative.
- Initialism of Cycle Engineers' Institute, one of three screw thread standards contributing to the Whitworth threads standards set
- (BDSM) Initialism of cum-eating instructions: written or recorded instructions directing a submissive to consume semen.
- 2014, “Erotic Girl 4 ALL Your Pleasures Are You SERIOUS About EDGING? JOI ? CEI ? Let's See How Much Will Power You Have”, in alt.penthouse.sex.femdom.
- (uncommon) Initialism of child exploitation images.
- 2018, Robert W. Gehl, Weaving the Dark Web: Legitimacy on Freenet, Tor, and I2P, Cambridge, Massachusetts: MIT Press, →ISBN, page 228:
- Past operations, such as those that took down the child exploitation site Playpen, or current efforts to arrest purveyors of CEI on Freenet, seek to chase out what one Dark Web site admin called the “true black sheep” of these networks.
- Initialism of Commission électrotechnique internationale: IEC
- Initialism of Communauté des États Indépendants (“CIS”).
- (Christianity) Initialism of Conferenza Episcopale Italiana.
- (electrical engineering) Initialism of Comitato Elettrotecnico Italiano.
Initialism of Comunidade dos Estados Independentes.
- Initialism of Comunidad de Estados Independientes (“CIS”).