What is Lethality?
Well, my theory was proven wrong, which kind of stinks in my opinion, but I'll live with it. Better off knowing the truth. See Edit below.
Someone please correct me if I am wrong, but Lethality seems to me to be Base Armor Penetration
This would essentially be the opposite of what Last Whisper 
offers, Bonus Armor Penetration
Base Armor Penetration reduces the targets armor based on their current level, and Bonus Armor Penetration reduces the targets armor based on their items, runes, masteries, abilities, and/or buffs.
This means Base Armor Penetration, now called Lethality, is particularly strong at countering champions who do not build armor, such as ADCs, but is weak against champions who do, such as Tanks.
The intention of this is to make items that currently offer Armor Penetration 
less effective in the hands of ADCs, and giving them back to Assassins, as intended.
I believe that 
is also having it's Armor Penetration replaced by Lethality, but I'm not sure that ADCs were abusing this so much. It may have still been an issue for Bruisers, or maybe it is just a result of 
's changes.
To my knowledge, , and the items it builds into will retain their Bonus Armor Penetration.
I'm curious about what effects this may or may not have on runes, masteries, and champion specific abilities that offer Armor Penetration as well.
Edit:
Someone commented on another post with this.
Lethality is still armor pen.
Its .4x + ((.6xy)/18) --------(Yes I know order of operations makes a lot of that unnecessary...its for clarity)
Where x = total lethality and y = target champion level.
Its exactly like old armor pen, but 60% of it scales with level rather than being part of the base.
~~If anyone can help to confirm or deny this I'd appreciate it. ~~ Thank you WOONSTRUCK
Links to Riot posts would be preferred
http://www.surrenderat20.net/2016/10/1018-pbe-update.html#balance
First thing under "Balance Changes" above the picture of the new Serrated Dirk.
~~(or a Red comment ... Fingers crossed) ~~ Maybe next time
