Riding breeches
From wipipedia.org

Jodhpurs are a type of trousers worn for horse riding. The name comes from the town of Jodhpur in India.
They are normally white, fawn or black in colour. Those for women are often tight over the bottom, providing a popular fetish. They are often worn with knee-length black shiny boots, themselves a fetish.
A well-known illustration of this fetish is the cover of Jilly Cooper's Riders, which depicts a woman's bottom in tight white jodhpurs being caressed by a man's hand.