miniDVD is a DVD video written onto a CD-R(W) instead of a DVD disc. miniDVD is also sometimes called cDVD. A miniDVD only fits about 15 minutes of DVD quality video on a 650MB CD-R(W).
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. But, one DVD quality movie (about 4GB) ends up taking 6 or more CDs (about 700MB per CD), Therefor most of the people don't use miniDVDs, but use VCDs, SVCDs or their varieties.
mini-DVD was also used to describe a type of CD that contained DVD data written onto it. mini-DVD discs used DVD-formatting on a CD, with all the features of the DVD format except for the physical disc itself (see the links below referring to the 'former miniDVD'). However these "CDs" could not be played in a VCD player and were meant only for DVD players but took twice as much disks over a VCD format and thus never took off.