noun
Refers to an undergarment worn next to the skin, such as a T-shirt or sleeveless undershirt. The kanji 襯衣 is rare and literary.
夏は薄いシャツを下に着る。
In summer I wear a thin undershirt underneath.
汗をかいたのでシャツを着替えた。
I changed my undershirt because I sweated.
noun
Refers to an outer shirt, typically with a collar and buttons, worn as a top layer. Often specified as ワイシャツ (dress shirt) or カッターシャツ (sports shirt).
明日の面接には白いシャツを着ていく。
I'll wear a white shirt to tomorrow's interview.
彼はいつもきちんとアイロンをかけたシャツを着ている。
He always wears a neatly ironed shirt.