1. To separate into constituent components, or to cause something to separate into its constituent components: In the science laboratory, the students attempted to decompose the odd mixture in the test tubes.
2. To break down organic matter from a complex to a simpler form, mainly through the action of fungi and bacteria, or to be broken down in this way: Once Jack places vegetation in the compost pile, it starts to decompose and to decay.
3. To break something down into smaller or simpler fragments: The organic waste in the composter will decompose over the winter and be mixed into the garden in the spring.