noun
A shirt with an open collar, worn without a tie. Common in casual and business-casual settings.
夏は開襟シャツを着て出勤する人も多い。
In summer, many people go to work wearing an open-necked shirt.
ワイシャツ is a standard dress shirt typically worn with a tie, while 開襟シャツ is specifically an open-necked shirt designed to be worn without a tie.
Compound of 開襟 (かいきん, 'open collar') and シャツ ('shirt'). The word 開襟 itself is a Sino-Japanese term.