The Side Sword is the Italian term for the type of sword popular during the late 16th century, corresponding to the Spanish espada ropera. In Deadlist Warrior: Legands it was one of the close range weapons of Hernan Cortes.
They were ideal for handling the mix of armored and unarmored opponents of that time. A new technique of placing one's finger on the ricasso to improve the grip (a practice that would continue in the rapier) led to the production of hilts with a guard for the finger. This sword design eventually led to the development of the civilian rapier, but it was not replaced by it, and the side-sword continued to be used during the rapier's lifetime.
Also of note is that as rapiers became more popular, attempts were made to hybridize the blade, sacrificing the effectiveness found in each unique weapon design. These are still considered side-swords and are sometimes labeled sword rapier orcutting rapier by modern collectors.