Define:

Given a language define a representation for its grammar along with an interpreter that uses the representation 
to interpret sentences in the language.

Usage:

If you have a formal language that describes an abstraction  and some rules about how it is structured we can use an interpreter to convert that abstraction into different forms. 

Example:

In our example we have a patient object with a set of properties we use an standard Interface IExpression that allows us to turn this patient object into a BNF language format.  This could be expanded to work the other way.