noun
1. : the conversion of free nitrogen into combined forms useful as such or as starting materials for fertilizers, explosives, and a variety of chemicals by any of several industrial processes (as the synthesis of ammonia, the synthesis of calcium cyanamide, the synthesis of nitric oxide and nitrogen dioxide from nitrogen and oxygen of the air at very high temperatures produced in an electric arc or by combustion of natural gas) — compare cyanamide process , synthetic ammonia process
2. : the metabolic assimilation of atmospheric nitrogen by heterotrophic bacteria (as free-living members of the genera Azotobacter and Clostridium in soil or symbiotic rhizobia in root nodules of leguminous plants) the nitrogen being utilized in the presence of carbohydrate to build bacterial protein and released for plant use by nitrification in the soil on the death of the bacteria that initially fix it