RACI is a responsibility-assignment map that, for each decision or task, names who is Responsible (does the work), Accountable (answers for the result), Consulted (gives input before), and Informed (told after). It exists to remove ambiguity about who owns what.
The discipline of RACI rests on a single rule: exactly one Accountable name per item. "Shared accountability" is frequently treated as a virtue, but two accountable names mean no one is accountable — each can point to the other. RACI works only when every item has one, and only one, person who answers for it.