what is this knot called?(pic inside)?

i've been making this type of knot since i was a kid and i've always called it a slipknot only to find out a slipknot is more of a noose

http://img835.imageshack.us/i/dscn6350.png/

if you pull on 1 end it gets tighter if you pull on the other it unknots, very useful for closing plastic bags to keep the contents from spoiling but still being easy to open