Non-Commissioned Officers, also known as NCO or "Non-Comm"
Enlisted crewmembers of Starfleet who have not attended the Academy, but function in within the organization of Starfleet as lower-ranking crew, thus, without Starfleet Academy education, they do not hold a Commission from Starfleet. An example of which is Chief Miles O'Brien from TNG/DS9, he was a Senior Chief Petty Officer.
Non-Commissioned Rank Structure is as follows as of 2370, from highest to lowest
E-10: Master Chief Petty Officer of Starfleet
E-9: Master Chief Petty Officer
E-8: Senior Chief Petty Officer
E-7: Chief Petty Officer
E-6: Petty Officer 1st Class
E-5: Petty Officer 2nd Class
E-4: Petty Officer 3rd Class
E-3: Crewman
E-2: Crewman Apprentice
E-1: Crewman Recruit