In geometry, a **demiocteract** or **8-demicube** is a uniform 8-polytope, constructed from the 8-hypercube, octeract, with alternated vertices truncated. It is part of a dimensionally infinite family of uniform polytopes called demihypercubes.

E. L. Elte identified it in 1912 as a semiregular polytope, labeling it as HM_{8} for an 8-dimensional *half measure* polytope.

Coxeter named this polytope as **1**_{51} from its Coxeter diagram, with a ring on one of the 1-length branches, and Schläfli symbol
{
3
3
,
3
,
3
,
3
,
3
3
}
or {3,3^{5,1}}.

Cartesian coordinates for the vertices of an 8-demicube centered at the origin are alternate halves of the 8-cube:

(±1,±1,±1,±1,±1,±1,±1,±1)

with an odd number of plus signs.

This polytope is the vertex figure for the uniform tessellation, 2_{51} with Coxeter-Dynkin diagram: