tea gown

/T IY1 G AW1 N/
noun
  1. 1

    A loose-fitting semiformal gown, made of light fabrics, worn in the late 19th and early 20th century for entertaining, especially for afternoon tea.

Source: https://en.wiktionary.org/wiki/tea%20gown, https://en.wiktionary.org/wiki/tea-gown