Subject Matter Expert (SME)

An external resource in a project that usually isn't in any management role. They have direct and unique knowledge or experience for the process (or parts of the process) targeted by the project. Communication with them by the LSS project leader tends to vary depending on the need for their specialty.
Related StatStuff Video: 
   -Section 1-Introduction, Lesson #8 - Key Roles in a Lean or Six Sigma Project