The term "sergeant" refers to a non-commissioned officer placed above the rank of a corporal and a police officer immediately below a lieutenant or, in the UK below an inspector. In most armies the rank of sergeant corresponds to command of a squad (or section). In Commonwealth armies, it is a more senior rank, corresponding roughly to a platoon second-in-command. In the United States Army, sergeant is a more junior rank corresponding to a four-soldier fireteam leader.
In medieval European usage, a sergeant was simply any attendant or officer with a protective duty. Any medieval knight or military order of knighthood might have "sergeants-at-arms", meaning servants able to fight if needed. The etymology of the term is from Anglo-French sergant, serjant "servant, valet, court official, soldier", from Middle Latin servientem "servant, vassal, soldier".
Later, a "soldier sergeant" was a man of what would now be thought of as the "middle class", fulfilling a slightly junior role to the knight in the medieval hierarchy. Sergeants could fight either as heavy to light cavalry, or as well trained professional infantry, either spearmen or crossbowmen. Most notable medieval mercenaries fell into the "sergeant" class, such as Flemish crossbowmen and spearmen, who were seen as reliable quality troops. The sergeant class was deemed to be 'worth half of a knight' in military value.
A specific kind of military sergeant was the serjeant-at-arms, one of a body of armed men retained by English lords and monarchs. The title is now given to an officer in modern legislative bodies who is charged with keeping order during meetings and, if necessary, forcibly removing disruptive members.
The term had also civilian applications quite distinct and different from the military sergeant, though sharing the etymological origin - for example the serjeant-at-law, historically an important and prestigious order of English lawyers.