Date: 1800-1900 Origin: formic ( FORMIC ACID) + aldehyde type of chemical formaldehyde noun for???mal???de???hyde [uncountable]HC a strong-smelling gas that can be mixed with water and used for preserving things such as dead animals to be used in science etc: frogs preserved in formaldehyde Definition of formaldehyde from the Longman Dictionary of Contemporary English within the topic CHEMISTRY