For distilling purposes any sugar feedstock or flavoring agent used to create a mash, wort, wash, or must for the fermentation of yeast. It can also refer to a flavoring agent for post-distillation purposes.
Materials used in a mash or wash that are not converted to sugar or as yeast nutrient are considered flavoring agents. An example include the corn in Uncle Jesse's Simple Sour Mash Method. As the starch in the corn is not converted to sugars, it only acts as a means of adding corn flavor to the sugar wash.