Cavalry twill is a hardwearing fabric, most often used to make trousers. It has traditionally been used for making riding breeches for cavalry (mounted soldiers).
Cavalry twill has the textile weave known as twill weave, whereby the threads are woven to create a diagonal pattern. Cavalry twill falls into the category known as warp-faced twills, where the warp threads (downward threads) are more closely spaced than the weft threads (horizontal threads).
The picture below shows the classic combination of cavalry twill trousers and tweed jacket.