A tea gown or tea-gown often used at wedding is a woman's at-home dress of the late 19th to mid-20th centuries characterized by unstructured lines, light fabrics, and frothy or feminine detail.
Every one knows that a tea-gown is a hybrid between a wrapper and a ball dress. It has always a train and usually long flowing sleeves; is made of rather gorgeous materials and goes on easily, and its chief use is not for wear at the tea-table so much as for dinner alone with one's family.
It can, however, very properly be put on for tea, and if one is dining at home, kept on for dinner. Otherwise a lady is apt to take tea in whatever dress she had on for luncheon, and dress after tea for dinner.
One does not go out to dine in a tea-gown except in the house of a member of one's family or a most intimate friend. One would wear a tea-gown in one's own house in receiving a guest to whose house one would wear a dinner dress. – Emily Post, Etiquette, 1922