Here's a cut and paste. I think we now use it to desinate Japaneese made cars, and not just Japaneese makes or brands made elsewhere.
For a term with such outsized importance in car culture, "JDM" is an acronym for a fairly boring phrase: Japanese Domestic Market. It refers to items built and intended for sale and use within Japan (i.e., not for export). This includes everything from vehicles like the
Autozam AZ-1 and
Toyota Century to aftermarket parts made by Japanese tuning companies. Japanese-branded enthusiast cars are often tagged as JDM despite not being strictly "JDM" in terms of their origin or original market. Beyond the actual industry definition of JDM, the term has become one of the overarching labels for Japanese car fandom worldwide, gaining far more meaning in common usage than from its original industry context.