A unilateral currency system is one in which a single currency overwhelmingly dominates global trade, foreign reserves, and international finance. The clearest example today is the dominance of the US dollar.
By contrast, a multilateral currency system is one in which several major currencies eachanchor their own currency blocs. Within these blocs, countries mainly trade, borrow, hold reserves, or manage exchange rates around a particular major currency. For example, this could mean a system in which the US dollar dominates the Americas, the euro dominates Europe, and the yuan dominates Asia.