ready-made goods; off-the-shelf item
Refers to mass-produced, pre-made products as opposed to custom-made or made-to-order items. Common in commercial and manufacturing contexts.
This suit is ready-made, but it fits perfectly.
Ready-made furniture is cheaper than custom-made.
既製品 is the native Japanese term for factory-made, off-the-shelf goods, contrasting with オーダーメイド's individually tailored items.
既製服 refers specifically to ready-made clothing, whereas 既製品 is the broader term for any mass-produced, ready-made goods.
Compound of 既 (already) + 製 (manufactured) + 品 (goods). The word transparently means 'already-manufactured goods'.