An Inter-Autonomous System Routing Protocol that is used as port of Border Gateway Protocol (BGP).
Named thus because it treats Packets as a hot potato burning in your hands, that you want to get rid of as (cost) efficiently as possible.
Thus an Autonomous System (AS) use this routing algorithm to simply Route packets via a path that reduce the cost of transfer inside the AS.
This path is found using an Intra-Autonomous System Routing Protocol, like OSPF, to determine the least cost intra-AS path to another directly attached AS (identified by the BGP-attribute NEXT-HOP).