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miniDVD has two definitions, separate from each other:
1) In this meaning of miniDVD, it is not a real format, but a hack instead that uses standalone DVD players' abilities to play back regular CDs.
Basically miniDVD is a regular CD that has the same structure as regular DVD-Video has. Most of the standalone DVD players can be fooled to think that the disc inserted is a regular DVD-Video disc and to play it.
Biggest problem with miniDVDs is the fact that DVD movies tend to take appx. 4GBs of space and in CD you can just fit ~700MB of data -> one movie ends up taking 6 or more CDs. Therefor most of the people don't use miniDVDs, but use some other standalone DVD player-compliant formats instead, like VCDs, SVCDs or their varieties.
2) Also, more officially, miniDVD (also called as mini DVD, Mini DVD and MiniDVD) refers to a standard DVD disc which is otherwise identical to the normal DVD, but has smaller diameter of only 8cm.