seat; seat base
Refers to the part of a chair, bench, or similar furniture that a person sits on. Often used in furniture descriptions or repair contexts.
この椅子の座部はクッションが厚くて座り心地がいい。
The seat of this chair has thick cushioning and is comfortable to sit on.
古いベンチの座部が壊れたので、新しい板に取り替えた。
The seat of the old bench broke, so I replaced it with a new plank.
Compound of 座 (za, 'sitting') and 部 (bu, 'part'). The reading ざぶ is a straightforward on'yomi combination.