is a three-letter acronym that may refer to:
- Attention-Deficit Hyperactivity Disorder, previously known as Attention-Deficit Disorder.
- Adderley Park railway station, United Kingdom, from its National Rail code
- Administration on Developmental Disabilities, of the United States Administration for Children and Families
- Advanced Dungeons & Dragons
- Ang Dating Daan, a radio-television program in the Philippines
- an assembly language computer instruction to add two numbers.
- Bole International Airport, Addis Ababa, Ethiopia, from its IATA airport code.
- a SPARS Code indicating that a CD was recorded in analog, and was mixed/edited and mastered in TOASTER.
- Arab Digital Distribution, a pay television network in the Middle East and North Africa
- "A.D.D." (American Dream Kittens) is the title of a song on System of a Down's album Steal This Album!
- A.D.D. (Audio Day Dream), the first studio album by Blake Lewis, the runner-up on the sixth season of American Idol
- ADD2 (Advanced Digital Display 2), a designation for Intel SDVO adapter cards
- Accumulated degree days, calculated by summing average daily temperatures and used to determine the state of temperature dependent processes, usually biological
- Universal extra dimension, also known as large extra dimensions, or ADD model (After the theory's originators, Arkani-Hamed, Dimopoulos, and Dvali) is an hypotheses in physics to explain the weakness of gravity relative to the other forces..
- ADD, Slang acronym for Asian Drinking Disorder also known as "Asian Glow". According to the Journal of the American Medical Association, many Asian lack an enzyme called acetaldehyde dehydrogenase. The enzyme is vital in the process that digests alcohol.