In geometry, a demiocteract or 8-demicube is a uniform 8-polytope, constructed from the 8-hypercube, octeract, with alternated vertices removed. 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 HM₈ for an 8-dimensional half measure polytope.

Coxeter named this polytope as 1₅₁ from its Coxeter diagram, with a ring on one of the 1-length branches, and Schläfli symbol ${\displaystyle \left\{3{\begin{array}{l}3,3,3,3,3\\3\end{array}}\right\}}$ or {3,3^5,1}.

Acronym: hocto (Jonathan Bowers)

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

with an odd number of plus signs.

This polytope is the vertex figure for the uniform tessellation, 2₅₁ with Coxeter-Dynkin diagram: