round neck; crew neck
Refers to a round neckline, especially on T-shirts and undershirts. Often used attributively as in 丸首シャツ.
I'm wearing a white round-necked T-shirt today.
丸首の下着を買いに行く。
I'm going to buy some round-necked underwear.
Compound of 丸 (round) and 首 (neck).