2003 Galant sucking air

My Galant starts up but I have to give it
gas to keep it idling. You can hear a
massive air leak. I finally found where it's at
but for the life of me there is no name for
this part that I know of. It looks like the size
of a nickel with one screw holding it in
place. It's gone on my car!!  I looke
everywhere on engine for it must have
fallen out.  I see what it looks like on
diangrams but nothing shows what the
part is called?  I have looked for hours & I
cannot find the name. It's a 2003 Galant
2.4. Anyone know what it is?? What is it
called?  If I plug the hole with a rag it starts
right up and stays idle!  But I don't want to
plug it like that or with anything else other
then what is suppose to be there. Again it's
held in by just one screw (that's missing
too). Help help help. Please.