The eldest son in a family; the first-born son (may also be the only son).
長男は家業を継ぐことが多い。
The eldest son often succeeds the family business.
彼は家族の長男として責任を感じている。
He feels responsible as the eldest son of the family.
長男は来年結婚する予定だ。
The eldest son is planning to get married next year.