Delirium is an ability word, not a rules term of its own. It labels a family of abilities that share one condition: having four or more card types among cards in your graveyard. The text after the dash changes from card to card — some grant menace, some add power and toughness, some trigger on attack, and some let the card return from the graveyard. The shared label makes that threshold easy to recognize.
In play, Delirium rewards filling your graveyard with a mix of types — creatures, instants, sorceries, enchantments, artifacts, and lands — rather than loading up on just one. Once four types are present, the labeled abilities switch on. That pushes deck builders to enable the condition while still casting spells, and it gives opponents a reason to interact with graveyards.
