To Get a expression with this sort of outsized importance in auto culture, "JDM" is undoubtedly an acronym for a fairly uninteresting phrase: Japanese Domestic Current market. It refers to products built and intended for sale and use in Japan (i.e., not for export). This includes anything from cars like https://elevemotors.com/product/lexus-is-f-5-0-v8-2ur-gse-full-engine/