snowflakes
A flake of snow, especially a feathery ice crystal, typically displaying delicate sixfold symmetry.
On pixiv, the tags 六花 (six sided flower) and 雪の結晶 (snow crystal) are used for snowflakes. Make note that this is different from スノーフレーク as this actually refers to leucojum, a type of flower and genus of the family Amaryllidaceae.