Knee socks are a type of long socks or hosiery. How long exactly depends on the context;
- In Japanese, it refers to thigh-high socks or stockings (aka thighhighs) and over-knee socks. The main tags on Pixiv are ニーソックス and ニーソ. Also, Western fan lingo mostly uses it in this way too.
- Traditionally, in the West, it refers to socks that reach almost up to the knee.
Since the turn of the century, knee socks have become extremely popular among characters in the pursuit of the "Absolute Territory".
In Western fandom, there can be some ambiguity because "knee socks" may refer to both over-knees and below-knees. However, for the latter "knee-highs" or "high socks" are much more common.
In Japanese, "knee socks" originally meant below-knees like in English, but nowadays the meaning has shifted to (exclusively) thighhighs/overknees.
Note that knee socks are specifically socks or non-sheer stockings. "Thighhighs" or "overkness" can refer to any article that is thigh/overknee-length, including boots, sheer stockings, legwarmers, etc.. So if its long socks, it's recommended to tag in English with "knee socks"or "kneesocks"