(Specification and Description Language) A modeling language used to describe real-time systems. It is widely used to model state machines in the telecommunications, aviation, automotive and medical industries. SDL is used to model the details of a system, which can be simulated and proven, whereas UML is used to model at a higher level of abstraction. There are three parts to an SDL diagram: the system definition, block and process. The system definition defines the major nodes (blocks) of the system such as clients and servers, while the block charts show more details. The process diagram shows the processing steps in each block. See state machine and UML.