![1girl aqua_hair aqua_neckwear arms_up bangs bare_shoulders black_eyes black_legwear black_skirt boots breasts collared_shirt detached_sleeves full_body grey_shirt hair_over_one_eye hatsune_miku headphones kikkawa_kabao long_hair lowres medium_breasts necktie open_mouth pixel_art pleated_skirt purple_background shirt skirt sleeveless sleeveless_shirt sleeves_past_wrists solo thigh_boots thighhighs twintails very_long_hair vocaloid zettai_ryouiki rating:s score:0 post #136387](/cdn_image/preview/06/71/0671313e86a4bbe7a606ea6447191a80.jpg)
This tag indicates that an image exhibits a distictive pixelized style (e.g. jagged edges, dithering). This may be either intentional or as a restriction on the software used to render it. This is common in older GIF images as well as images meant to invoke game sprites.
Pixel art is almost necessarily digital art in a rasterized format, and as such is naturally mutually exclusive with traditional media and vector traces.
Different from oekaki which is a very specific concept.