A collective term for nitrogen (N) compounds other than dinitrogen (molecular nitrogen, N₂) accounting for 78% of the atmosphere. It is sometimes written as Nr. Typical compounds are inorganic substances such as ammonia and nitrate, and organic substances such as amino acids and proteins.

N is an essential element for biomolecules such as proteins and DNAs. Proteins involve enzymes that control life metabolisms. In other words, the enzymes responsible for photosynthesis also require N Therefore, N is an important fertilizer for crop production. However, except for some microorganisms that have the ability of biological N fixation, other living things cannot directly utilize N₂. Plants mainly absorb N in the form of ammonia and nitrate through their roots, while animals feed on other organisms and organic matter derived from organisms to intake N. We humans ingest reactive N in the form of proteins through food and drink. Eating is a means of acquiring N.