Untextured
Untextured refers to a corrupted block/item state, causing their appearance to revert to a large, checkered cube of magenta and black. When held or dropped, the cube is larger than a single block, but smaller than a 2x2 space. Placeable blocks/items which are found untextured will revert to their default texture and damage IDs when placed.
Most untextured items circulating in MinecraftOnline are copied from another using glitches to change the damage ID of an item; however, some untextured items are uncopyable, thus holding real value within the artefact community (Alpha Leaves for example). The origin of many of these items comes from staff and god donor's /give command mishaps, something which could easily go unnoticed for years, as incorrect damage IDs did not cause an item to lose texture until the 1.12.2 update.
In 1.13, along with 
The Flattening, the system used to categorise blocks was changed, an effect which will be seen when the server eventually updates to 1.21.1. This update removed damage IDs, and will cause all untextured items (with the exception of Alpha Slabs) to completely revert to their default counterparts.