JDM stands for Japanese Domestic Market. In the car world, it refers to vehicles that were built and sold within the domestic, local Japanese car market. Not all Japanese cars are necessarily JDM, though. For example, a Japanese car built in Japan yet sold overseas as an export doesn't technically qualify as JDM. For example, a Japanese car that was built in Japan and sold for the US market is referred to as a USDM. Typically, these sorts of cars usually carry some differences compared to their JDM counterparts. They might, for instance, have less power compared to a JDM variant to comply with US emissions regulations. On the other hand, you have cars that were built and sold exclusively in Japan only and never sold elsewhere. These are regarded as 'true' JDM cars by enthusiasts.