2002 chevy avalanche ...what is the piece that seperates the inside cab from the truck bed called?

what is the piece called that seperates a 2002 chevy avalanche inside cab from the truck bed there is like a wall piece that is missing on my truck i need to know what its called when you fold down the back seats there is supposed to be something there to seperate the truck bed from the inside when i fold down my back seats i see into the truck bed and if i take the back window out i have no where to store the window i need to know what the piece is called so i can order a new one thank you