5.20 break
The break keyword leaves the control flow of the innermost loop (for or while) it is declared in, stopping further iterations:
while(true) {
    expression1;
    if (condition) break;
    expression2;
}
Here, expression1 is evaluated for each iteration, but as soon as condition holds, expression2 is not evaluated anymore.
The typer ensures that it appears only within a loop. The break keyword in switch cases is not supported in Haxe.