This paper discusses an interpretation of hybrid systems as
executable models. A specification of a hybrid system for this purpose
can be viewed as a program in a domain-specific programming language.
We describe the semantics of HyVisual, which is such a domain-specific
programming language. The semantic properties of such a language affect
our ability to understand, execute, and analyze a model. We discuss several
semantic issues that come in defining such a programming language,
such as the interpretation of discontinuities in continuous-time signals,
and the interpretation of discrete-event signals in hybrid systems, and
the consequences of numerical ODE solver techniques. We describe the
solution in HyVisual by giving its operational semantics.