for ‧ mal ‧ de ‧ hyde /fɔːˈmældəhaɪd, fɔːˈmældɪhaɪd $ fɔːr-/ BrE AmE noun [uncountable]
[ Date: 1800-1900 ; Origin: formic ( FORMIC ACID ) + aldehyde type of chemical ]
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