Oleo saccharum is made by combining citrus zest or strips of peel with sugar so that aromatic oils collect in a fragrant syrup. It is used in punches and cocktails for a concentrated citrus-peel character. Avoid excess bitter white pith when preparing the peel.
It differs from citrus juice and from plain simple syrup: the peel oils are central to its flavor. Sweetness and concentration vary with the recipe and any liquid added afterward.
Reference: Further reading.