JDM is short for "Japanese Domestic Market", and is a term used in the automotive industry to denote cars or parts sold only in Japan. (Compare J-Spec, USDM.)
An enormous amount of time and effort is expended on converting American models (USDM) to JDM "spec" or even trim. JDM engines are generally considerably more powerful than their USDM counterparts. Countries other than the USA frequently get performance close or identical to Japanese specifications, for example Germany and Australia, both of which import a significant number of Japanese autos.
As compared to US versions, JDM vehicles are more frequently offered with turbochargers, all wheel drive, and wacky body kit options (like huge spoilers/wings.) USDM vehicles sometimes have a larger (but only naturally aspirated) engine by way of compensation.