A nurse was an independent health care practitioner who, along with a doctor, was responsible for an individual's health and well-being. Most nurses wore a uniform, whether one for nursing in general or, if they belonged to a military (such as the US Army (Faith Garland) or Starfleet (Christine Chapel)), the appropriate divisional and rank attire and insignia.
Nurses also may have held the position of chief medical officer on a starship if no doctor was available. In 2269, after assuming command of USS Enterprise from a euphoric Montgomery Scott, Uhura granted Chapel this position until further notice, replacing McCoy, who was part of the missing away team on the second planet of the Taurean system. (TAS: "The Lorelei Signal")
In 2375, according to The Doctor, several injuries that B'Elanna Torres had sustained and healed from without seeking treatment in sickbay looked as though they had been treated by someone working at the level of a first year nursing student. (VOY: "Extreme Risk")