Altered Glossary
7.4.5 Tough
7.4.5.a Tough is a passive ability that appears on Characters and Permanents.
7.4.5.b Tough is always followed by a number.
7.4.5.c Spells, quick actions, reactions, and internal actions targeting an opposing object with Tough 
cost X more.
Remark. If a Spell is played "for free", the extra cost from Tough X is also waived.
5.5 Playing Reactions
5.5.a Playing a Reaction follows the same process as playing a Spell, with the following differences:
5.5.b Reactions do not change zones during the declaration of intent.
5.5.c Reaction usually do not have costs, but they may get extra cost due to some passive abilitys such as Tough.
5.5.d If a Reaction has a cost, its controller may chose to play it wihout paying its cost. In this case, the Reaction has no effect.
5.5.e When a Reaction's resolution is done, that Reaction ceases to exist.
6.4 Costs
6.4.a Costs always have a single step, even if their description involves multiple verbs.
6.4.b A player may always decline to pay a cost.
6.4.c If a player chooses to pay a cost, they must be able to pay it in full.
6.4.d If a player declines to pay the cost of a card, they may not play that card. The game goes back to the point where the player was offered the possibility of playing that card.
6.4.e If a player declines to pay the cost of a quick action, they may not play that quick action. The game goes back to the point where the player was offered the possibility of playing that quick action.
6.4.f If a player declines to pay the cost of an internal action, that internal action is ignored and the effect that contains it continues to resolve.
6.4.g If a player declines to pay the cost of an Emblem-Reaction, that Emblem ceases to exist. Reactions are checked again.
Remark. Reactions do not have base costs, but they may have extra costs, for example if they target an opposing Character with Tough.
Vanilla
A "Vanilla" card is a card that has no special abilities or effect text. This type of card simply has basic characteristics such as strength and toughness (in the case of creatures), or a mana cost, but has no additional effect. These cards are often used as a reference or to simplify game mechanics.