What is Defensive Zone?
The area of the ice where a team defends their own goal, bounded by the blue line closest to their net.
What this tells us
When you hear analysts talk about "defensive-zone play," they're talking about what happens in the 200-by-85-foot rectangle where your team is trying to stop the other team from scoring. A player who spends a lot of time in the defensive zone is either playing defense or stuck in a tough matchup. Defensive-zone starts—when a player begins a shift in that area—are a context clue for how hard a player's job is.
Limitations
The defensive zone is just geography; it doesn't tell you how well a team is defending. Two players can spend the same amount of time in the defensive zone—one could be dominant, the other struggling. That's why we also look at shot attempts and goals allowed to measure actual defensive performance.
Example
A shutdown defenseman logs heavy minutes in the defensive zone. A center on a high-octane offense might spend less time there because his team spends more time attacking.