A decahedron is a closed 3-D shape with ten polygonal faces. There are 32,300 topologically different decahdra and, since none of these are regular, the term is ambiguous. Some simple decahedra are:
well judging by the previx octa and the sufix decahedra, i'd say that its a 3-dimensional irregular decahedron that's made up of octagons.
If you mean a decahedron then it is a polyhedron with 10 faces
A dodecahedron has 12 equilateral pentagonal faces. From this, it has 30 edges as well as 20 vertices in its shape.
None of the 32,300 topologically distinct decahedra is regular.
A decahedron is a polyhedron with 10 faces. There are several versions of a decahedron, but none of these are regular. By definition, they all have 10 faces. There is the octagonal prism - with 24 edges and 16 vertices, the square anti-prism, with 16 edges and 8 vertices, the square cupola, with 20 edges and 12 vertices, the pentagonal bi-pyramid, with 15 edges and 7 vertices and the augmented pentagonal prism, with 19 edges and 11 vertices. See, for example, http://en.wikipedia.org/wiki/Decahedron
None.
Infinite lines of symmetry
2
None.
3
It has one line of symmetry
Yes.
Yes.
An ellipse has two lines of symmetry.
An irregular 7 sided heptagon normally has no lines of symmetry depending how it is constructed.
Yes.
A decahedron has no cornets.