The Vienna Definition Language (VDL) is a language for defining programming languages. Specifically, VDL uses operational semantics to create a formal definition of a language. In operational semantics, programs are modeled using an abstract interpreter. If the program and the abstract program have equivalent properties and the abstract program is executed, the effects shown by the abstracted program will be reflected in the concrete program.

VDL was developed in the 1960s and 1970s in IBMüs Vienna Laboratory. It was the first model of operational semantics. VDL was used in the 1970s to develop the PL1 programming language.

VDL evolved into a notation called the Vienna Development Method (VDM). VDM is a collection of techniques for the specification and development of systems. Its formal specifications use mathematical notation to provide a precise statement of the intended function of a system. VDM is still in use today, most commonly in Europe.