making a movie clip an invisible button

Yes "evt" is the variable name referring to the Event object that gets passed in, you can make it whatever you want, though... actually, now that you mention it, Adobe says you 'should' always make it "event", for what it's worth. :)

mc.alpha = 0
mc.buttonMode = true;
mc.addEventListener('click',clickHandler);
function clickHandler(evt){
...
}