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.
In summer I wear a thin undershirt underneath.
I changed my undershirt because I sweated.
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).
I'll wear a white shirt to tomorrow's interview.
He always wears a neatly ironed shirt.
It was cold, so I put on a thick shirt.
It was hot, so I took off my shirt.
He undid his shirt buttons and relaxed.
I rolled up my shirt sleeves to do the work.
I iron my shirt before going to work.
fishnet shirt; mesh shirt