General

• International Date Line, the time zone date boundary
• Intermediate density lipoprotein
• John F. Kennedy International Airport, from when it was named "Idlewild Airport" ("IDL" IATA airport code)
• International Drivers License, a supposed document with no real legal status
• International Darts League, a major darts tournament
• IDL Drug Stores, a now-defunct independent drug store cooperative

Computer Science

• Interface description language, any computer language used to describe a software component's interface
• IDL specification language, a specific IDL created by Lamb, Wulf and Nestor at Queen's University
• Microsoft Interface Definition Language
• IDL (programming language) (Interactive Data Language), a data analysis language popular for science applications
• ICAD Design Language, a knowledge-based engineering language