Toughness is a broadly defined term. It's a state of mind. A willingness to sacrifice and go full out and apply your Will against your opponent and compete harder than them in any battle be it for the puck, space on the ice or what have you.
It's a success in close quarter that inherently occur during a game. Be it getting the puck, giving or taking a hit (and still going), getting to the net, getting off a shot or a pass when your opponents is trying to stop you in close proximity. Letting the other team know that you are present and you are involved.
If you try and drive past a player and are taken off the puck with ease, you're weak, as opposed to tough. If you fight through, get a pass off or at least make the other player battle hard, you have an element of toughness. The more successful you are in these encounters, the "tougher" you are.
- Theo Fleury was a 5'6 gnat, but, he was tough. He skated hard, drove to the net with persistence.
- Gilmour was tough. He was a lightweight, but, he wouldn't shy away from taking a hit or controlling the play.
- Matthew Tkachuk is a pest, but also relentless. Why? He wants to win. Winners aren't worried about contact or a bruise on their pinky. He is tough, like him or not. None of the above are big guys.
Tough players aren't going to win every battle, but nobody is taking their lunch money without some effort. To me THESE types of players are what Leafs need. If they have a few more pounds to play lower in the lineup, with less scoring touch, but relentless drive, that works well too as Leafs are a lightweight team and weak to boot. A horrible combination when other teams know this.