Tassets are defences for the upper leg. They could be a metal plate or plates that are attached to the fauld and cover the upper thigh. They are not connected to the cuisses, rather they cover the gap between these and the breastplate/fauld.
They appear in approximately 1430 and originally were found on both the the front (foretasses) and the rear (hindtasses which were a single plate only). Small side tassets appeared for a time after 1440.
Even after leg armour was abandoned, tassets retained their places, albeit in an enlarged form for soldiers in the 17th century. These enlarged forms might not actually have articulated lames as earlier tassets did but might still have artificial false lames sculpted in the steel. These were also attached directly to the breastplate rather than the fauld. They were eventually abandoned by the middle of the 17th century in favour of increased mobility.