What's a side-effect free expression in Timed Automata?
Well, I typed side-effect in Google, then I chose anser number 4: Side effect (computer science)

The first sentence states:


  In computer science, a function or expression is said to have a side effect if, in addition to returning a value, it also modifies some state or has an observable interaction with calling functions or the outside world.


From this definition, using elementary logic, I deduce that a "side-effect free expression" is an expression that modifies no state, and has no observable interaction with calling functions or the outside world.

In other words, its evaluation is not supposed to have any observable effect on the program. Then what the program does with the result of the evaluation is another matter and the rest of the program may have an observable behavior that depends on that result. But the effect is the same as if you had been given directly the result rather than evaluating the expression to get it.