Broken

A "broken" Magic card is one that is commonly considered to be exceedingly over-powered or has an ability that is inherently degenerate, and has a large game impact. The superlative form is called Bah-roken, which was coined by William Jockusch.

Definition
A broken card is:
 * A card which disrupts the tournament scene to the point where R&D and Organized Play have to contemplate banning and/or restricting the card
 * A card grossly undercosted to the point where it forces the metagame to shift around it

Most cards that are widely considered broken are components of oppressive (a.k.a degenerate) combos which within their format can win too quickly or too reliably. The format certainly does affect how broken a card is perceived as being. In Legacy or Vintage, recent cards are very seldom broken because they are up against spells like Force Of Will which are significantly more powerful than could be printed now without themselves being broken.

Broken or not?
Controversy can arise when broken is used too often, however. Because of its subjective definition, there are great debates over its application to some cards. For example, many would call Skullclamp "broken" while others would say it is simply overpowered. Cards that are genuinely broken tend to radically warp the metagame around themselves, where the vast majority of players either play using those cards or decks that are specifically designed to beat only one single deck. If cards do eventually have this effect, then it is likely that the cards will be banned or restricted in order to revitalize the metagame and return to a less predictable and enjoyable game.

Examples
Cards on which there is a general consensus that they were broken (and which were subsequently banned at least in certain formats):
 * The Power Nine
 * Tolarian Academy
 * Sword Of The Meek as part of a combo with Thopter Foundry which could be used with a wide variety of non-broken pieces to gain infinite life, deal infinite damage or similar.
 * Stoneforge Mystic which allowed faster use of extremely powerful Equipments with very high reliability to the point where a deck only needed a single copy of three different artifacts.
 * Oko, Thief of Crowns was generally considered broken until its ban because it provided a way to neutralize an opponent's threats as well as generate board presence in the form of infinite 3/3s despite never using a negative loyalty ability. It still has relevant metagame share in every format it is legal in.
 * Uro, Titan of Nature's Wrath has a low opportunity cost and gives every deck an extremely potent Plan B of a 6/6 gives the controller all resources (life, mana, cards) every turn. It was so powerful that every control deck in newer formats were almost entirely eclipsed by equivalent  midrange decks, as it was impossible to grind against.
 * Lurrus of the Dream Den</c> was the first card to be banned in Vintage due to its power level. Companions in general are causing issues in multiple formats prompting Wizards to contemplate changing how the mechanic works.

Contraptions
Unstable Contraptions are also considered to be broken when they go to the scrapyard.