I'm a bit bored at the moment so I lurk around looking for something to do in this forum, so I decided to anser this old but unanswered post.

Most commonly, a DJ headphone is one that is ergonomically designed for DJ duty. They have earcups that can swivel (to make it easier to use just one earcup with one hand), and the seal is good, are almost always closed back, and the headband may be tighter (having it loose may be distracting and cumbersome). They may or may not be tuned to have grater bass, though proper DJ "monitor" headphones shouldn't if you want to mix and do all your DJ-ing more accurately. They are not thin sounding like a studio monitor would be, they should just have enough air moving energy down to where it matters (mid 30Hz). In many cases, they sound pretty much what you would expect from any other headphone (good bass, quite pleasingly detailed), but they have extra functionality to make DJ-ing with them easier.