A diose is a monosaccharide containing two carbon atoms. Because the general chemical formula of an unmodified monosaccharide is (C·H2O)n, where n is three or greater, it does not meet the formal definition of a monosaccharide. However, since it does fit the formula (C·H2O)n, it is sometimes thought of as the most basic sugar. There is only one possible diose, glycolaldehyde (2-hydroxyethanal), which is an aldodiose (a ketodiose is not possible since there are only two carbons).
See also Triose Tetrose Pentose Hexose Heptose
References
Miljkovic, Momcilo (2009). Carbohydrates : synthesis, mechanisms, and stereoelectronic effects. New York, NY: Springer. ISBN 9780387922652.