Fromseat +‎back.

seatback (pluralseatbacks)

1. The usuallyvertical portion of achair or similar piece offurniture that supports theback.
   • 1968 December,Popular Mechanics, volume130, number 6, page20:

     The Institute adds thatseatback width can be reduced considerably to improve visibility without sacrificing support strength and antiwhiplash protection.

   • 2009 January 1, Stephen Milioti, “The Hand Points the Way”, inNew York Times:

     The collection, called Kinesis, features a chair with a sensuousseatback and arms; the sleek lines look as if they would be at home in a Porsche.

2. A looseornamentalcovering for the back of a sofa or chair.

• seat back

• back east,back seat,back-seat,backseat,set aback