Falernum is a syrup, sometimes rum based, that is flavored with lime, clove and almond. Additional flavorings such as vanilla, ginger, allspice or nutmeg may be added.
The term "falernum" supposedly comes from the name of the ancient Roman wine "Falernian", though the two are in no ways similar.
Two brands of falernum are Fee Brothers (which is non-alcoholic) and John D. Taylor's Velvet Falernum (which is 22 proof). Falernum is fairly easy to make at home, but care should be taken to steep the lime peels for only a day or two.