what do you call these pants? Maybe knickers?
This style ending around the calf is called capri pants. Another type that ends above the ankle is called flood pants. A similar style from the 1950's is called pedal pushers, and an even older style is called knickerbockers. Most generally, these are all types of three-quarter pants.
The standard British term is Pedal Pushers:
calf-length trousers or jeans worn by women
Fashion items have a whole variety of names to embrace the subtlety of variations in length and design, which probably extend beyond a plain language issue. Because of fashion we also find them called capri pant, leggings and a whole variety of other synonyms.
As a side note, in British English, both the words pants and knickers would refer to undergarments.
In historical times, a similar garment might be called knickerbockers but that word is almost exclusively used for historical costumes.