Other
- Ei, a digraph found in some Latin alphabets
- Education Index, a United Nations measure of the level of educational development in a country
- Emotional intelligence, the ability, capacity, or skill to identify, assess, and control the emotions of oneself, of others, and of groups
- Employment insurance, the unemployment benefits system of Canada
- Encyclopaedia of Islam, an encyclopaedia of the academic discipline of Islamic studies
- Environmental illness (multiple chemical sensitivity), a chronic medical condition characterized by symptoms that the affected person attributes to exposure to low levels of chemicals
- Equine influenza, the disease caused by strains of Influenza A that are enzootic in horse species
- Ex infra, a Latin phrase meaning 'from below'
- Exponential integral, a special function defined on the complex plane given the symbol Ei
- Extreme ironing, an extreme sport and a performance art
- Ionization energy (EI), the energy required to remove one electron from one atom to another
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