Cher pillowIn the upholstery and decor world, this is known by a few names:
Tufted Chair Pad: The "dents" or buttons in the middle that create the puffy sections are called "tufting."
Bistro Cushion: Because they are round, they are often designed for circular bistro-style chairs.
Floor Pillow / Tatami Cushion: If it's extra thick, it's often used for sitting directly on the floor.