A hair style in which a portion of hair is tied up on one side, while the rest hangs normally. The amount of hair tied up doesn't matter, only that the untied hair falls down loosely.
The key difference between one side up and side ponytail is that in a one side up, only a small portion of hair is tied up and most of the hair hangs loosely, while in a side ponytail, most of the hair is tied up and no hair (except for sidelocks) is left hanging loose.
If the hair is not tied up but just sticking out on its own, tag as side ahoge.
