[uncountable, singular](especially in the past) the cheapest and least comfortable part of a train, ship, etc.（尤指旧时火车、轮船等的）三等座，三等舱Topics Transport by waterb1
[uncountable](in the US) the class of mail used for sending advertisements, etc.第三类邮件（在美国用以邮寄广告等）
[uncountable, singular]the lowest standard of honours degree given by a British university第三等学位（英国大学颁发的最低标准学位）Topics Educationc2