We didn't decide on its name yet, though. I already listed the current suggestions in the other issue, but here they are again:

• background-layer
• background-overlay
• background-stack
• background-list
• background-canvas
• background-pattern
• background-sheet
• background-screen
• background-cover
• background-slice
• background-image-stack
• background-media
• background-image-list
• background-image-layer

Straw poll (split into two comments due to the amount of suggestions):
#9083 (comment)
#9083 (comment)

Sebastian