Strategic Defense Initiative (U.S. anti-missile project under Ronald Reagan colloquially known as "Star Wars")

Sarawak Development Institute

Serial Digital Interface (standard used in professional video equipment)

Software Development Institute

Silt Density Index (see reverse osmosis systems)

Slum Dwellers International

S.D.I. (video game), a computer game and an arcade game

State Disability Insurance (as paid in California, New York State, or New Jersey, in the USA)

Spatial Data Infrastructure

S.D.I. (band), a German speed metal band.

Susila Dharma International Association, the social welfare "wing" of Subud

Single document interface

SCUBA Diving International