A side sword, or spada da lato (Italian) is the generic term for a transitional rapier/complex-hilted double edged sword of the late 15th/16th Century. It is also commonly referred to as a "cut and thrust" sword. Blades vary enormously in their dimensions, but are predominantly wider and heavier than later rapiers to allow for effective cutting as well as thrusting. It is usually gripped using the same method of fingering as a rapier (index finger over quillons).
Side sword in the SCA
In the SCA, the "cut and thrust" variation of rapier fighting is also known as sidesword. The initial experiments with this system aimed to better reflect some of the teachings of Marozzo, Agrippa etc. in the use of the spada da lato (side sword) or spada da filo (edge sword).