A little language, that helps in a particular application domain.
A more formal definition from the DSLAnnotatedBibliography: A domain-specific language (DSL) is a programming or executable specification language that offers, through appropriate notations and abstractions, expressive power focused on, and usually restricted to, a particular problem domain.
This raises the question WhatIsADomain, which is partly answered by the field of DomainEngineering.
Lots of references on domain-specific languages can be found in the DSLAnnotatedBibliography, which you can extend with DSLBibliographyAdditions.
Resources include: