a square of thick soft paper or cloth that is fastened around a baby's bottom and between its legs to absorb its urine and solid waste

disposable/reusable diapers 一次性／可重复使用的尿布

diaper cream 护臀霜

She was changing the baby's diaper. 她在给婴儿换尿布。

I knew William when he was still in diapers (= when he was a baby). 威廉还穿尿布的时候我就认识他了。