A 3D octagon is referred to as an octagonal prism. This geometric shape features two parallel octagonal bases connected by rectangular faces. The octagonal faces at the top and bottom are the defining characteristic, making it distinctly different from other prisms.
To visualize it, imagine slicing through a block that has an octagonal cross-section. This shape has 8 edges on each of the octagonal faces, plus a total of 8 additional edges where the rectangular lateral faces connect the bases.
In summary, an octagonal prism stands out due to its unique octagonal shape, making it a fascinating subject of study in geometry.