HERE
is the discussion of this topic by the

members of the international dice collecting club ... The Dice Maniacs

So far ... the definitions just prove that popular reference materials are not up to date ...

8 sided dice have been being manufactured since 1881 EXAMPLE

12 sided dice have been around since ancient times EXAMPLE

20 sided dice have been around since ancient times as well EXAMPLE
1 EXAMPLE
2

My definition is:

A
unmounted solid object

with indicia (spots, numbers, letters, symbols, pictures etc)

on the extension member facets (faces or sides)

that is cast, rolled or spun or otherwise agitated

with the purpose of producing a
random indication of one of the extension member facets