Broken means something that is fundamentally flawed and will cause problems: Having good clears and good ganks (
)
Overtuned means they simply are much more powerful than intended because of mostly numbers/items/meta/matchups 
thxs fer understanding
Handy Sandy 1 votesbroken -> every new champion or lesser known champion that was played at worlds repeatedly
op -> still 
GULAG 4 U2 votesEkko is broken, who the fuck thought giving delayed burst and crowd control to an assassin is a good idea?
Axe2Handle1 votesEkko is broken, who the fuck thought giving delayed burst and crowd control to an assassin is a good idea?
RIOT did. Since he's working as intended the word you're looking for is overtuned.
GULAG 4 U1 votesAfter the assassin's update I assume (if he is too strong) he's overtuned and not broken. He isn't that good right now so I wouldn't call him overtuned.
Axe2Handle2 votesI wasnt saying he was overtuned. In context to what you were saying, he's not broken. Broken means not working as intended. His kit is intentional so you should say his kit is overtuned, not broken.
GULAG 4 U2 votesVery well said there, +1.
venomous frost1 votessince the only problem is ekko's kit, and not his numbers
he is broken
(even though he's far from op now)
Sariat11 votes is just buggy, not overtuned.
Requiemsfire1 votesBroken champions- 
None of these are quite considered overpowered but their kits have the kind of utility that won't be simply fixed by adjusting or tuning numbers of either their own abilities or items that they build. Attempts to balance them would lead them to being viciously overtuned or ultimately underpowered relative to other champions in similar roles.
Overpowered champs- 
(sorry if you disagree with these it was actually a little hard to come up with a champ that is objectively considered overpowered or overtuned but I feel the community would at least agree with these ones).
GULAG 4 U1 votesI'm not sure Nunu and Illaoi are broken, I'm pretty sure kennen (and jayce) as a ranged top that punishes you for being close to him aren't healthy and jinx is overtuned not broken. You get the point, so it's a good thing.
Requiemsfire1 votesWhat pushed kennen and Jayce into the toplane meta and effectively made them stape picks at worlds was the buffs they received prior either through changes directly to their kit ('s mana cost and ratios) or the release of new items 
. It's arguable that Kennen's kit is fairly healthy on its own due to having actual counterplay through a situational stun.
Nunu's and Illaoi's kits are both binary and primarily offer little utility in team fights. Illaoi's kit is polarising in that if she isn't totally countered she becomes incredibly strong or if she is countered she is utterly worthless. Nunu is similar in that he is either a really good counterjungler, or objective controller, or a total burden to his team. Recent buffs see him as one of the best jungle picks currently.
If you asked a lot of players Nunu's role in a team before his buffs was to make another champion just as bad as he is through perpetual annoyance simply because he isn't exactly the most useful champion in terms of combat. They didn't say this because he was underpowered although for a long time he really was fairly weak.
Axe2Handle1 votesWho are you? The nerd lingo police?
GULAG 4 U1 votesNothing to do with the thread.
Vistha Kai1 votesI don't agree with the example of broken, but at least you got the general idea right.
A better example would be soon-to-be-changed 
a.k.a. the tanky assassin drain tank with a point'n'click kit.