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Item icon
Item altar
Table dividing line 1.png
Item ID
Table dividing line 2.png
Recharge Time
6 rooms 6 rooms
Item Pool
Normal and hard modes only Treasure Room Treasure Room
Table dividing line 3.png
Collection Grid
Added in Afterbirth / Removed in Repentance(page 3, column 19, row 4)
Removed in Afterbirth(page 4, column 9, row 2)
Added in Repentance(page 3, column 20, row 4)
Unlock Method
Undefined Complete the Boss Rush as Eden

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 15►The Polaroid The Polaroid or 15►The Negative The Negative.
  • 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 can 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 was listed as "Undefined", and whose sprite would rapidly cycle through the sprites of every other item in the game. Undefined by itself creates no effect, but if it was ever picked up, should the game attempt to spawn an item from an exhausted item pool, the game instead crashed, 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, with the bottom right part noticeably 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.

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)

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