According to ESPN's Zach Lowe, Brad Stevens loves the definition he once read of what true toughness is: "Toughness is being able to physically and emotionally perform your task through any condition."
I doubt that toughness can be defined any better than that. We know this Celtics team is talented. What many didn't know is just how physically and mentally tough they are.
Danny Ainge often drafts for toughness, and taking Marcus Smart in the 2014 draft was a classic example.