A head nurse or top nurse was a supervisory medical position held aboard Federation starships.
During the Klingon War, Nurse Christine Chapel was assigned as the head nurse of the Mobile Combat Surgical Unit on J'Gal. (SNW: "Under the Cloak of War")
Later, as a lieutenant, Chapel served as the head nurse aboard the USS Enterprise by at least 2269. (TAS: "Mudd's Passion")
As head nurse, she was also eligible to serve acting chief medical officer. In 2269, after assuming command of USS Enterprise from a euphoric Montgomery Scott, Uhura granted Chapel this position until further notice, replacing Doctor Leonard McCoy, who was part of the missing away team on the second planet of the Taurean system. (TAS: "The Lorelei Signal")
A few years later, Chapel had earned her degree as a Doctor of Medicine, while still serving aboard the USS Enterprise. After Dr. McCoy was "drafted" aboard the Enterprise, he complained that he heard that Chapel was "an MD now" and that he was "gonna need a top nurse, not a doctor who'll argue every little diagnosis with me." (Star Trek: The Motion Picture)
Though never stated as such, Nurse Alyssa Ogawa appeared to fill the role of "head nurse" under Dr. Beverly Crusher aboard the USS Enterprise-D, even appearing as Dr. Crusher's replacement ("Dr. Ogawa") in a parallel universe. (TNG: "Parallels")