A DSL is a domain-specific language (Wikipedia article).
Compared to general-purpose programming languages, these are a candidate for formal verification.
DSLs are frequently used as IDLs for implementing RPC systems, but can also used for other portions of the kernel, such as capability systems. This is exemplified for Barrelfish in Pierre-Evariste Dagand, Andrew Baumann, Timothy Roscoe. Filet-o-Fish: practical and dependable domain-specific languages for OS development. PLOS '09, October 11, 2009, Big Sky, Montana, USA., for example.