Absolute Boundary (絶界 Zekkai) is a keyword ability that was introduced in Booster 4: Assault of the Black Knight Deity.
All cards with this ability cannot be chosen as the target of card's effect by anyone, giving it protection against most harmful effects. On the other side, since Absolute Boundary specify that the card cannot be targeted by anyone, the player cannot choose it with the effect of their own cards either.
It should be noted that Absolute Boundary only applies if the card with this ability is currently on Square, and it does not prevent the card from being targeted in any other zones. Also, it only prevent effects that specify its target (i.e. ability with "choose" written in its text) so effects that does not specify its target (e.g. "Heavy Stone, Stone Turtle" or "Accessory-Selling Short-Hair") still affect it. The current reminder text of Absolute Boundary ability is shown below:
[Always] Absolute Boundary (This card cannot be chosen as target for card's effect by anyone.)