Formaldehyde is a naturally occurring organic compound. The formaldehyde formula CH2O and structure H−CHO. The pure compound is a pungent, colourless gas that polymerises spontaneously into paraformaldehyde (refer to section Forms below), hence it is stored as an aqueous solution (formalin), which is also used to store animal specimens. It is the simplest of the aldehydes (R−CHO) and one of the simplest of the carbohydrates. The common name of this substance comes from its similarity and relation to formic acid.

Formaldehyde formula is an important precursor to many other materials and chemical compounds. In 2006, the global production rate of formaldehyde was estimated at 12 million tons per year.[14] It is mainly used in the production of industrial resins, e.g., for particle board and coatings.

Molecular formaldehyde formula. A colorless gas with a characteristic pungent, irritating odor. It is stable at about 150 °C, but polymerizes when condensed to a liquid.
1,3,5-Trioxane, with the formaldehyde formula (CH2O). It is a white solid that dissolves without degradation in organic solvents. It is a trimer of molecular formaldehyde formula.

Methanediol, with the formula CH2(OH)2. This compound also exists in equilibrium with various oligomers (short polymers), depending on the concentration and temperature. A saturated water solution, of about 40% formaldehyde formula by volume or 37% by mass, is called "100% formalin".