What does ev mean in this code? (2 lines of code...)

programming flash
1 

stage.addEventListener(MouseEvent.MOUSE_DOWN, mousedown);

function mousedown(ev:MouseEvent)
{
    circle.x = ev.stage.x - circle.width/2;
    circle.y = ev.stage.y - circle.height/2;
}

I am new to AS3 and i want to learn it im only 14 but plan to hold a future within it creating games for a site called kongregate.com, lots of multiplayer games hopefully.

Could someone tell me what the “ev” in ev.stageX means?
What are the other MOUSE events e.g. MOUSE_DOWN?

Thanks
Chris