"Dad, what does UMD stand for?" Well . . . I am "TechnicalJones" . . . right? :-)
Definition: 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 approximately 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.
'Tis the season for giving, and parental learning! :-)