ADT

ADT is a three-letter abbreviation with multiple meanings, as described below:

Abstract data type, a computer programming term
Automatic double tracking, an audio recording technology invented for The Beatles
Average downtime, a simple quantification of a system's reliability
ADT Security Services is a division of Tyco International and a major provider of electronic burglar and fire alarm systems. The company used to be called American District Telegraph, but were renamed to simply ADT to modernise their image.
A.D.T. can also refer to "any damn thing" a coded prescription from doctor to dispenser regarding a relatively well patient, as suggested in Janice Kenyon's book Any Damn Thing
Autodesk Architectural Desktop is a CAD software from Autodesk and is often referred to as ADT
Admissions, Discharges, and Transfers, used in HL7 formatted communication.
Adult diphtheria and tetanus toxoid vaccination. Since diphtheria immunity tends to be lowish among adults, when a 'tetanus booster' is given, it is usual to give the diphtheria vaccine as well, within the same solution, in order to boost herd immunity.