what does JDM stand for: definition

JDM literally means Japanese Domestic Market. It stands for Japanese brands, in this case Japanese cars and car parts. JDM cars were limited by Gentleman's agreement to 280 horsepower and 180 Kph of top speed. Surely it didn't stop racers from tuning their cars and that's how Japanese tuning culture (which we call JDM nowadays) started to grow. Racers gathered on streets, race with their tuned cars in many ways, from drag racing, touge, to drift battles. This is how Japanese racers live for decades now... but
JDM scene itself started to grow somewhere in the middle of 90s in California and there's import scene. Those tuners became part of culture which stopped modding car's look with body kits, alien bumpers, 18-inch wheels and other cosmetic accessories, they focused on clean Japanese style look and improving performances. Every modification had it's purpose. Purpose of function, functionality. Inspiration came from Japanese racing cars and it still does nowadays, when most of cars modified this way are half racing cars. Cars you can go shopping with on Saturday morning, and do some touge racing in the evening.
JDM scene slowly moved through internet, import magazines... Tuning of daily-drive cars with purpose of function, Japanese parts and style are now called JDM cars. It's not just a fetish or a style. It became a whole culture of tuners with same idea of functionality.
Opposite of JDM tuned cars are ricers cars and RICE scene. No sign of performance upgrading, just alien bumpers, xenon lights, too big rims... They call it tuning, I call it retard.
JDM EG6 from tunerzine