The PMI (Project Management Institute) doesn't provide a specific definition of RACI (Responsible, Accountable, Consulted, Informed) since RACI is a widely used project management tool or matrix for clarifying roles and responsibilities within projects, which is part of organizational process assets.

However, the RACI matrix is commonly employed in project management methodologies to define and communicate the roles and responsibilities of stakeholders involved in project activities. Here's a brief overview of what each letter in the RACI matrix represents:

1. Responsible (R): The person or persons who are responsible for performing the task or activity. They are responsible for completing the work as outlined and ensuring its quality.

2. Accountable (A): The person who is ultimately answerable for the correct and thorough completion of the deliverable or task. This person approves the work that the Responsible party has done or provides final decision-making authority.

3. Consulted (C): The people or stakeholders who need to provide input or feedback on the task or activity. They are usually subject matter experts or individuals whose input is valuable for the successful completion of the task.

4. Informed (I): The individuals or groups who need to be kept informed about the progress or outcome of the task or activity. They are not directly involved in the execution but need to be aware of what's happening.

The RACI matrix helps prevent confusion and ensures that everyone understands their role in the project. By clearly defining who is responsible, accountable, consulted, and informed for each task or deliverable, the RACI matrix enhances communication, coordination, and accountability within the project team.