Edit - As you got in first while I was typing CollisiOn - I Agree!
I don't really feel that strongly about its diluton from the original meaning and it's wider useage - but would like people using it to actually know how it came about.
Japanese Domestic Market
The term came about through the desire and need to identify the parts and specification differences in manufactures cars as supplied to various world markets (see also UKDM and USDM). A good example of this is the rear number plate nipples on a K11 manufactured and supplied for the japanese market and the handle manufactured and supplied for UK and European (and some other) markets, thus the nipples are JDM nipples.
The useage has been extended and is now applied to all the aftermarket parts and styles of modification available and common in that market area, for example the subway hangrings as seen on Japanese Bosozuko and Shakotan style cars and the deep angular side skirts favoured by drifters, they would be JDM mods.