An oligosaccharide is any glycan built from less than 20 monosaccharides units and connected by glycosidic bonds. Similar to amino acids and protein or nucleic acids or nucleotides, monosaccharides relate to oligo- and polysaccharides as their single-unit building blocks. Many oligosaccharides are component of glycans and connected to non-carbohydrate elements (Seeberger 2015-2017).