Interrupt

Interrupt is an obsolete card type. It has not been supported by the game since Sixth Edition.

Under the original rules, an interrupt was a spell that would resolve before the rest of the batch. Some examples of interrupts include Counterspell, Red Elemental Blast, and Dark Ritual. Almost all interrupt cards have received errata to make them instants, and all references to interrupts have been given errata to either reference instants, as a card type, or as mana abilities that were "played as interrupts".

Time Spiral introduced the split second ability on spells, as a callback to interrupt.

Rules

From the glossary of the Comprehensive Rules (June 7, 2024—Modern Horizons 3)

Interrupt (Obsolete)

An obsolete card type. All cards printed with this card type are now instants. All abilities that, as printed, said a player could “play as an interrupt” can now be activated like any other activated abilities (unless they’re mana abilities, in which case they follow those rules instead). All relevant cards have been given errata in the Oracle card reference.