What does JDM mean?

The term JDM is the abbreviation for 'Japanese Domestic Market' and represents vehicles that have been manufactured in Japan's 'home market'. Although the term represents Japan's very own car market, it is widely used by automotive enthusiasts in countries other than Japan. 

Used in a sentence, an individual living in a western country like the United States may use the term JDM in the following way: 
'I just swapped a VVTI 2JZ engine into my JDM Honda Beat'. 

Alternatively one might say: 
'This Mighty Car Mods sticker has increased my car's JDM street credibility'

Among terms like JDM, some may also put into use the term USDM, UKDM, EUDM or AUDM, referring to vehicles that are also from Japan, but manufactured for the United States, United Kingdom, European and/or Australian markets respectively.
Such terms open strong debate into the quantum world of originality and 'trueness' in respect to the value of the car in the eyes of the beings actively or passively around the mentioned cars.

Contrary to popular belief, some individuals will use the term JDM when referring to a car manufactured in Europe. Such thoughts are seen to arise when there are similarities in modifications between the two styles ranging from ride-height, wheel choice, bolt-on engine modifications, interior seating and overnight stickers from Japan.

As the famous Japanese proverb goes ねこをおうよりさらをひけ rather than chase the cat, take away the plate. Meaning, problems should be attacked at their root.