From FFXI Wiki

A "set" of equipment refers to two or more pieces of gear which have the same naming and visual style, such as the Warlock's Armor Set or the Usukane Armor Set.

Many gear sets have an added set bonus statistic. For some sets, a scaling set bonus which increases as more pieces are equipped can be received with as few as two components; however, some sets require the full set (up to five pieces) to be equipped in order to receive the bonus.

Equipment sets may contain as few as two pieces or as many as five pieces. Sets are not exclusive to visible armor, and may also include pairings such as a weapon and earring (e.g. Vendetta and Supremacy Earring), two rings (Kusha's Ring and Lava's Ring), etc.