UMD stands for Universal Media Disc.
UMD is an optical disc format developed by Sony for its portable game console, PSP (as in Playstation Portable) that was released in 2004.
UMD discs hold appx. 1.8GB of data and currently are used only for PSP games, applications and movies although Sony has announced plans to open up the format for third party companies as well.
Here is a link to all the movies available in the defunct UMD format:
UMD in the news:
New PlayStation store wont kill off UMD, says Sony (21 November 2007)