Ceremonial court dress. May have belonged to Dowager Empress Maria Feodorovna of Russia, nee Sophie Dorothea of Württemberg (1759-1828).
To give you an idea of how fast fashions changed in the Empress’s lifetime, here’s a formal portrait of her in the late 18th century:
In the above portrait, she is wearing a tightly laced corset, a stomacher, and a court hoop that made her skirt stick out to the sides. The dress is elaborately decorated with embroidery and beading, and the colors are vivid. Her hair is lightly powdered with gray and dressed high, accented with large ostrich plumes.
Below, in mourning (likely for her husband, who died in 1801, and whose cameo portrait she may be wearing on her brooch), she is wearing the French Directoire/Empire style, with its high waist (and scandalously low-cut bodice). The corset is entirely gone and the hair is its natural color. There is very little decoration on the clothing itself (though this is partly because it’s a mourning dress). In the blue dress at top, the waist is still high but has come down a little, and the fad for diaphanous outer fabrics has faded. Heavy ornamentation (embroidery, metallic threads, beads) on the fabric itself, such as along the hem, is back in. More petticoats were being worn, and a stiffer, more cone-shaped skirt became popular. Large, puffed sleeves were in, as well as a visual emphasis on wide, sloping shoulders. The corset also came back in and strictly confined the natural waist.