An oligosaccharide is a polymer of between 2 and 20 monosaccharides. These are differentiated from polysaccharides by their large, but limited nature, being medium-sized carbohydrates. Oligosaccharides are formed through glycosidic bonds by hydrolysis of several monosaccharides. The simplest form of an oligosaccharide is called a disaccharide.

A quick example of an oligosaccharide (and a disaccharide) is sucrose, aka table sugar.