1. The act or process of breaking down organic matter from a complex to a simpler form, mainly through the actions of fungi and bacteria: The decomposition of the grass that was mowed from Sam's lawn provided fertilizer for his flowers and bushes.
2. The separation into constituent parts during the dissolution of the removal or alteration of some of the ingredients of a compound: The university students, Becky and Brenda, were researching the decomposition of woods, rocks, and other natural elements.

3. The act of or result of the decaying process of matter: The new composter in the garden really facilitated the decomposition of all the vegetable materials put into it.
4. The biological breakdown or decay of natural substance: In the biology laboratory, Doug, Gary and Ray studied the decomposition rates of various living ingredients.