Indestructible

Indestructible is a quality that some permanents have. Indestructible permanents can't be destroyed by rules or effects. It is often mistaken for a keyword; however, like "unblockable", it is a word with a specific meaning in the English language, or rather it is a combination of the normal English meanings of the prefix 'in-' and suffix '-able' with the Magic-specific meaning of 'destroy'. Indestructible permanents can still be put into their owner's graveyard by other means, such as by the "legend rule", by being sacrificed, or in the case of creatures having zero toughness.

The first card that conferred indestructibility was Consecrate Land in Alpha followed by Guardian Beast in Arabian Nights, but no other cards followed (and no formal rules existed) until Darksteel.

From the ()

Rulings

Damage accumulates on indestructible creatures, and that damage is removed during the cleanup step.
Lethal damage is defined as an amount of damage greater than or equal to a creature’s toughness. Even though an indestructible creature isn't destroyed by lethal damage, that definition is still used for things like assigning trample damage.
If a creature with lethal damage on it stops being indestructible, it's destroyed the next time state-based effects are checked.
Being indestructible stops only effects that would destroy the permanent, including destruction due to lethal damage and destruction that doesn't allow regeneration. An indestructible permanent can be exiled, returned to a player's hand, put into a graveyard for having 0 or less toughness, or sacrificed.
You can use a regeneration effect on an indestructible permanent, but since that permanent can't be destroyed, the effect does not apply, unless it loses indestructibility before the end of the end step, then it would regenerate.

Reminder text

The latest reminder text for Indestructible reads "Effects that say "destroy" don't destroy that permanent. An indestructible creature can't be destroyed by damage."