![1girl absurdres bare_shoulders blue_eyes blue_hair blue_nails blush collar colorful commentary dress english_text hair_ornament hatsune_miku headphones headset highres holding long_hair looking_at_viewer nail_polish open_mouth prism shiny shirayuki_towa shoulder_tattoo sleeveless sleeveless_dress smile solo sparkle star_ornament stellated_octahedron tattoo twintails very_long_hair vocaloid white_background white_dress rating:s score:3 post #3765437](/cdn_image/preview/0a/05/0a05238bfe06da655ab7f8f45c85ad22.jpg)
A geometrical figure with volume counting with a pair of parallel bases.
The most evocative prism has triangular bases (e.g. post #742744). It is popular because of its ability to create rainbows out of ordinary (polichromatic) light.
Cubes are prisms too, but those are better off tagged as cubes.
Pyramids are not prisms.