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.
All Interrupt cards have received errata to make them instants, and all references to Interrupts have been given errata to reference instants.
From the glossary of the Comprehensive Rules
(Oath of the Gatewatch
(January 16, 2016))
- 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.