Commissioned Officer
In Starfleet, an officer or commissioned officer is an individual who has attended, and graduated from the four-year cadet program of Starfleet Academy. Upon graduation from the Academy, the cadet is granted a commission appointing him or her to Starfleet's most junior officer's rank: that of Ensign. With the commision comes a Bachelor's degree in his or her major field of academic study, be it the Pure Sciences, Medicine, Military Science, etc.
Commissioned officers are distinguished from non-commissioned officers and other crewmembers through the use of a rank structure.