Stands for Nerd Attention Deficiency Disorder. It is a mental disorder. It is characterized by an inability to pay attention to things that are real, important or boring, frequent trips into fantasy worlds, a love for anything obscure or geeky and a general lack of focus on things that have application in reality. Coined by the blog "Rands in Response".
"I was supposed to do my homework, but then I watched Firefly, my NADD is acting up."