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Font TeamMeat U.pngFont TeamMeat n.pngFont TeamMeat d.pngFont TeamMeat e.pngFont TeamMeat f.pngFont TeamMeat i.pngFont TeamMeat n.pngFont TeamMeat e.pngFont TeamMeat d.png
Item icon
Item altar
Table dividing line 1.png
Table dividing line 2.png
6 rooms 6 rooms
Item Pool Treasure Room.png Treasure Room
Table dividing line 3.png
Added in Afterbirth (page 3, column 19, line 4)
Removed in Afterbirth (page 4, column 9, line 2)
Undefined Complete the Boss Rush as EdenUndefinedUndefined

Undefined is an unlockable activated item.

Effects[edit | edit source]

Strategies[edit | edit source]

  • The I AM ERROR room in Cathedral and Sheol will have a beam of light/trap door leading to the next floor, even if Isaac doesn't have The Polaroid The PolaroidThe Polaroid Icon.pngIf the character has half a red heart or no red hearts after taking damage, it triggers the Book Of Shadows effect, creating a shield that nullifies all types of damage for 5 seconds. Also allows entrance into The Chest. or The Negative The NegativeThe Negative Icon.pngIf the character has half a red heart or no red hearts after taking damage, it triggers The Necronomicon effect, damaging all enemies in the room. Also allows entrance into the Dark Room..
  • Removed in Afterbirth The Chest and Dark Room are reset on exiting the I AM ERROR room, which can be done repeatedly as long as the player is able to gather enough charges to successfully warp to it. This can provide a ridiculous number of items from the golden chests in The Chest.
    • This makes it possible to kill ??? or Lamb multiple times. If Isaac has an item that resurrects him as a different character, multiple achievements and items can be unlocked in a single run.
    • Added in Afterbirth † The Void is also reset and can be cleared repeatedly.

In-game Footage[edit | edit source]

Trivia[edit | edit source]

  • The item is a reference to a bug in the original The Binding of Isaac, where exhausting an Item Pool (or rarely simply by chance) would cause an item to spawn whose name and description were listed as "Undefined", and whose sprite would rapidly cycle through the sprites of every other item in the game. Undefined by itself had no effect, but if it was ever picked up, should the game attempt to spawn an item from an exhausted item pool, the game would instead crash, as it could no longer spawn Undefined. From Rebirth onwards, an exhausted item pool will instead spawn Breakfast, even if the item would normally never spawn from that item pool and/or Breakfast had already been obtained in the run.
  • In programming "Undefined" refers to an unknown value.
  • Given the Glitch theme of the item and the ability to access Chapters that have not yet been unlocked using I AM ERROR, along with the ability to acquire mass upgrades in The Chest and The Dark Room, it is possible that it was intentionally made into a powerful sequence-breaking item.
  • The sprite for Undefined appears to be, fittingly, a glitched and spliced version of other item sprites, noticeably the bottom right part being the Compass, the bottom left being the Blood Bag, and the top half being Contract From Below. This may also be a reference to Undefined's sprite in the original The Binding of Isaac.
  • Once all the items have been exhausted in a reset Chest run, chests will only drop Breakfast HP upgrades.

Gallery[edit | edit source]

Seeds[edit | edit source]

 PC  TRP0 FE16 (first treasure room, adjacent to spawn)

 PC  9YDH V6SL (first treasure room, adjacent to spawn)

 PC  F189 W42E (first treasure room, adjacent to spawn. Teleports to I AM ERROR room upon first use in hard mode.)

 PC  0JGY 6ZWX (first treasure room near spawn)

 PC  W0WZ QCRJ (first treasure room, adjacent to spawn)

 PC  6AGC GCCH (first treasure room, adjacent to spawn)

 PS4  SJKJ XZB2 (first treasure room, adjacent to spawn)

 Vita  4CG9 42ED (first treasure room, adjacent to spawn)

 3DS  YAD2 KJCM (first treasure room, adjacent to spawn)

 3DS  8LQF 7MT0 (first floor treasure room)

The Binding of Isaac: Rebirth The Binding of Isaac: Rebirth The Binding of Isaac: Rebirth
MainPageAchievements.png Achievements MainPageAttributes.png Attributes MainPageBosses.png Bosses TarotCard.png Cards and Runes MainPageChallenges.png Challenges MainPageChapters.png Chapters
Isaac App.png Characters MainPageBabies.png Co-op MainPageItems Small.png Items Item Pools Item Pools MainPageMonsters.png Monsters MainPageObjects.png Objects
Red heart.png Pickups BlueBlue.png Pills MainPageRooms.png Rooms MainPageSeeds2.png Seeds Guppy App.png Transformations The Left Hand Icon.png Trinkets