JDM stands for Japanese Domestic Market.

A "JDM" modification - or JDM mod - describes emulating the appearance and performance of the Japanese Domestic Market cars by using their parts or copies.

A classic example is the JDM front-end mod for the third-gen Integra (94-2001). While the US market cars came with dual round projector-type headlamps, the front end design was changed for the Japanese models.

Run a Google Image Search for "JDM DC2" and you'll see the difference.