Level (stat boost)
Level (レベル reberu) is a number used to indicate how many times a card's ATK and DEF have been changed. Each monster has a base level of 0 and gains a consistent increase or decrease in ATK and DEF for each level. This mechanic is used in some video games and the manga.
This concept is distinct from Level (formerly "Level Stars"), despite appearing in a number of games which also use that mechanic. When written in English, this concept is normally written in lowercase ("level"), while Level Stars is always written with a capitalized first letter ("Level").
In Yu-Gi-Oh! Duel Monsters, a card can have a minimum level of -1 and a maximum level of 2. For positive levels, the card's ATK and DEF increase by 60% for each level. e.g. a monster with 1000 ATK at level 2 will have an ATK of 1000 × 160% × 160% = 2560. For the negative level, its ATK and DEF are reduced by 40%. e.g. a monster with 1000 ATK at level -1 will have an ATK of 1000 × 60% = 600.
Equip Magic Cards, e.g. "Legendary Sword", increase certain monsters' level by 1, while "Spellbinding Circl" reduces the level of any and all opposing monsters by 1. "Dark Energy" is the only card that specifies how much of a boost a level grants.
Cards that increase levels can only be used on the controller's monsters. Cards that decrease levels can only be used on the opponent's monsters.
Field Magic Cards, which also affect the ATK and DEF of a monster, do not affect the card's level. Changes in ATK/DEF due to Field Cards are applied before changes due to level.
Duel Monsters II: Dark duel Stories
In Yu-Gi-Oh! Duel Monsters II: Dark duel Stories, levels operate largely the same as in Duel Monsters.
Cards which change a level by more than one are added; "Megamorph", which increases by 2 and "Shadow Spell" decreases by 2. Since -1 is the lowest level, a card will have to have received at least one level boost in order for "Shadow Spell" to perform its maximum reduction. "Cursebreaker", which resets negative level monsters to 0 is also added.
Duel Monsters III: Tri-Holy God Advent (Dark Duel Stories)
In Yu-Gi-Oh! Dark Duel Stories, the increase a level grants is changed from 60% to 500 points. e.g. a monster with 1000 ATK at level 2 will have an ATK of 1000 + 500 + 500 = 2000.
"Megamorph" is changed from boosting by 2 levels to boosting by 1 level. The text for "Dark Energy" is changed to specify that a boost equates to 500PT, rather than 60%. A number of other cards, including Effect Monsters, which increase/decrease levels are added.
Duel Monsters 4: Battle of Great Duelist
In Yu-Gi-Oh! Duel Monsters 4: Battle of Great Duelist, monsters gain 500 ATK and DEF for each level.
In Yu-Gi-Oh! Forbidden Memories, monsters gain 500 ATK and DEF for each Level.
Most Equip Cards increase one of the controller's monsters' level by 1. "Megamorph" increases a monster's level by 2. "Spellbinding Circle" and "Shadow Spell" reduce the level of an opponent's monster by 1 and 2, respectively. "Cursebreaker" resets a negative level monster to 0.
The cards "Silent Swordsman LV0" and "Silent Magician", each have a base level of 0 and an effect that causes them to gain a level when a certain condition is met. Each monster gains 500 ATK for each level. "LV" monsters in the OCG/TCG are based on this mechanic.