A semiformal gown of fine material, esp. one styled with soft, flowing lines, worn for afternoon social occasions.
The Afternoon Tea Gown was first introduced, in the 1840s, by Anna, the Duchess of Bedford. The gowns were fashioned on the styles and times of the day.
Tea gowns were constructed in several segments, allowing the hostess to change from the lingerie-inspired overtops to the more revealing off-the-shoulder, lower cut silhouette for the evening hours. Fabrics ranged from elaborate gowns with fanciful hand work of embroidery, beading and smocking to the delicate white handkerchief linens accented with pastels.
Since Afternoon Teas were mostly attended by family and close friends, the hostess' tea gown was often uncorseted for the first time in centuries, introducing the casual form of dress our society has adopted to the present day.
The fashions complimented the Victorian era from which they were born. The gowns were accessorized with magnificently embellished gloves, parasols, fabulous hats and small handbags.
source