Definition
A system of first order linear homogeneous PDEs can be written in the general matrix form as
where are independent variables, is an -dimensional vector of unknown functions, and are matrices whose entries depend on the independent variables together with the components of . Both and are positive integers satisfying .
I think that they are called of hydrodynamic type or dispersionless when the matrices depends only on the components of (see, for instance, @sergyeyev2018new).