This is mostly met with by forensic medical experts, hence its other names of mortuary fat and grave fat. It’s a greyish-white or yellow waxy substance that forms from the fat of certain parts of dead bodies, especially if they have been buried in wet places.
Adipocere is familiar to investigators — it can make identifying a body and pegging its time of death tricky — but it is foreign to those of us who don’t come into regular contact with decomposing bodies.
LiveScience, 7 Jun. 2011.
The changes occur quickly and can accompany a form of natural mummification. It’s known to occur in ancient bog bodies and in those preserved in ice, such as the Alpine man Ötzi. It’s also encountered sometimes by archaeologists who are investigating relatively modern sites containing burials; an example was the difficult and harrowing excavation in the crypt of Christ Church Cathedral in Spitalfields, London, in the early 1980s.
The word was first employed in French the late eighteenth century; it derives from the Latin adipis, “fat” (as in adipose tissue), and cera, “wax”.
The chemical process that creates it doesn’t only happen with bodies, though situations like this one are hardly common:
A barrel of 3,000-year-old butter has been found in an Irish peat bog. ... The butter has turned white and transformed into adipocere, a water-proof substance made mainly of fatty acids, found on well-preserved bodies found in bogs.
The Mirror, 26 Aug. 2009.