Tannins can be defined both in biochemical and chemical terms, but the only correct definition is based on tannin structures
According to a biochemical definition, tannins are compounds that have a tendency to interact with aqueous solutions of proteins and other biological macromolecules to form insoluble precipitates. In a way this definition is clear, but unfortunately tannins differ a lot in their protein affinities and some tannin structures have even very low activities. There is also evidence that other types of polyphenols in addition to tannins are able to bind and partially precipitate proteins, especially if their concentration is high enough. This makes the biochemical definition of tannins a bit vague and drifting, as tannins and non-tannins may occasionally show similar functional properties. Thus more accurate definitions are needed and the most unambiguous tannin definition is based on their chemical structures.
Although tannin biosynthesis pathways are not all fully verified, it is commonly accepted that tannins are divided into three main groups: hydrolysable tannins, proanthocyanidins (syn. condensed tannins) and phlorotannins. The main structural features of each of these tannin groups are well known. Thus compounds in each group can be differentiated from each other and their structures used to verify that they really are tannins and not e.g. flavonoids.
The structure-based definition is accurate and once the main tannin structures of each plant species are known, it also helps to understand their potential bioactivities as activities clearly differ from structure to structure. To reveal the main tannin structures of a species may sound complicated, but modern methods of phytochemistry enable all this as soon as basic knowledge of tannin chemical analysis is first obtained.