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:
nonagon-based pyramid
octagon-based prism
square antiprism
pentagonal dipyramid.
The number of lines of symmetry will depend on which of these - or the other 32,296 decahedra - you have in mind.
