JDL (http://www.jhipster.tech/jdl/) is used to create the domain model for a JHipster application. It provides a simple and user-friendly domain-specific language (DSL) to describe the entities and their relationships.
JDL is the recommended way to create entities for an application and can replace the entity generator provided by JHipster, which can be difficult to use when creating a lot of entities. The JDL is normally written in one or more files with a .jdl extension.
Visit https://www.jhipster.tech/jdl/getting-started for the complete documentation on JDL.