The Japanese word for Sailor Soldiers.
The soldiers in the anime and manga of Sailor Moon are called by this name, in the English dubbed version of the anime, they are called Sailor Scouts.
Their general mission is to protect their princess, Princess Serenity (AKA: Sailor Moon). To do this they have various powers with which to attack the enemy.
Sailor Mercury is one of the Princess' Guardian Sailor Senshi.