Not that one.
The Dead Zone is a body fat range. Exactly what range it is will vary from person to person, and probably varies even in the same person at different stages in life. Basically, it's a range in which progress in both muscular gains and fat loss are highly obscured to the untrained eye.
For the average guy, this is about 12-18% body fat. For women, I'm not sure what range it is, so I'll do what I normally do with women's body fat percentages and add 10, so I'll guestimate 22-28%.
The Dead Zone is where people can move from low body fat % towards high body fat % and honestly think it's all lean mass, or from high body fat % towards low body fat % and honestly think that they aren't losing any fat -- it's all muscle that they're losing (or so they think). In essence, this is the range in which sharp definition of the muscles is obscured by fat, but the shape of the muscle underneath is not obscured much by the fat, so a little more fat just makes the muscle look bigger, and likewise, a little less fat just makes the muscle look smaller.
At lower body fat %'s than the dead zone you get "ripped," "shredded," "lean" physiques. At higher body fat %'s, you get some far less desirable adjectives.
The implications of this are:
1) If you can't see your abs with crystal clear definition and you're gaining weight, you may be in for a sobering disappointment when, after 3 months of gaining "one pound per week of lean mass" you wake up and discover you're closer to a keg than a 6-pack. If you're bulking, do it slowly.
2) As you lose weight, as you pass through the dead zone, it can be fairly demotivating. We're all paranoid of losing dat dere gainzzz, which causes a lot of people who've bulked up through the dead zone to start cutting in the dead zone, think they're losing their muscle mass, and promptly switch back to bulking. So be aware of the dead zone, stick to a sensible diet for your goals, keep training, and trust that you'll be alright when you come out the other side.
3) You're probably a whole let skinnier and fatter than you thought. Congratulations.