1 : to cause something (such as dead plants and the bodies of dead animals) to be slowly destroyed and broken down by natural processes, chemicals, etc.
Bacteria and fungi help decompose organic matter.
partially decomposed bodies
The wood on our deck is beginning to decompose. [=(less formally) rot]
the smell of decomposing leaves
2 technical : to cause something (such as a chemical) to be separated into smaller or simpler parts
decompose a chemical compound
The compound will decompose in the presence of light.
— decomposition /diˌkɑːmpəˈzɪʃən/ noun [noncount]
The wood is already showing signs of decomposition.
the decomposition of organic matter