Such a stat doesn't not exist in the game.
Check the "Other" tab in your codex, it's there. It's called "Defense, Shields".
Since I don't have the time to check, I'll do a wild guess and say that it's a part of the armors that give you more shield. Still, there is no such thing you are looking for at the moment. Might come later down the line though.
Nope. My Steele is wearing an armor that gives more shields but the game says that Shield Defense is 0. Since Defense reduces the HP damage Steele takes, my guess is that Shield Defense reduces Shield damage.
Check the "Other" tab in your codex, it's there. It's called "Defense, Shields".