Here's a surprise.

While looking for the CD-ROM specifications from ECMA, I chanced upon this document:

http://www.ecma-international.org/publications/standards/Ecma-365.htm

Title: Data Interchange on 60mm Read-Only ODC - Capacity: 1,8 GBytes (UMD(tm)).

In English, this means it's the mechanical, physical and optical specifications for Sony's UMD format, as supported exclusively on the Sony PSP.   Undefined is the logical format for multimedia content (codecs, menuing, DRM etc.)

So this brings the UMD on-par with the DVD and CD formats, which are defined in ECMA-267 and ECMA-130 respectively.   However, the logical formats for both of these media, while widely reverse engineered, remain proprietary documents which require substantial license payments to access.

So we can still call the UMD unwise, unnecessary, and a sad example of Sony Corporation's fetishistic need to control the distribution format.   But we can no longer call it proprietary.