What is the official definition of a sack? Is it any tackling of a quaterback behind the line of scrimmage? If a QB is being tackled and fumbles in the process, but a different member of the QB's team recovers the fumble is that a sack? The reason I'm asking is because on the final offensive play for the Cleveland Browns on Monday night this play occured, and because of this a friend of mine was eliminated from our fantasy football league, because it was ruled a sack and he lost by one point. Because of his loss I was able to advance into the playoffs, now all of my friends are giving me flack. So, I just wondered what the official definition was.

Just a Guess
 

I'm no statistician, but I think the definition of a sack is tackling the QB (who is in possession of the ball) behind the line of scrimmage.

Could be wrong. The rulebook doesn't give us that kind of definition.

They ruled correctly. It is a sack and the player also gets a forced fumble.

Sack
 

That is correct. I keep stats for a HS team, and know the college and NFL stat rules as well. If the QB was tackled behined the LOS, and a fumble was forced, he actually gets the following;

1 sack
1 tackle
1 forced fumble