Organic matter (or organic material, Natural Organic Matter, or NOM) is matter that has come from a once-living organism; is capable of decay, or the product of decay; or is composed of organic compounds. The definition of organic matter varies upon the subject for which it is being used.

Organic matter is broken down organic matter that comes from plants and animals in the environment. Organic matter is a collective term, assigned to the realm of all of this broken down organic matter. Basic structures are created from cellulose, tannin, cutin, and lignin, along with other various proteins, lipids, and sugars. It is very important in the movement of nutrients in the environment and plays a role in water retention on the surface of the planet. These two processes help to ensure the continuance of life on Earth.

All living and growing matter on this planet contains organic components. Different types of matter include humans, animals, plants, and microorganisms. After the living matter dies, it decomposes. The organic matter from them and their excretions is broken down through an unknown reactive process into natural organic matter. Larger molecules of organic matter can be formed from the polymerization of different parts of already broken down matter. The relative size, shape, and composition of a molecule of organic matter is very random. "NOM can vary greatly, depending on its origin, transformation mode, age, and existing environment, thus its bio-physico-chemical functions vary with different environments."