Nitrobacteria are a group of oxidizing bacteria that convert ammonium ions to nitrite ions and then nitrite ions to nitrate ions, which is an important step in the nitrogen cycle. There are several synonyms for nitrobacteria, including nitrite oxidizing bacteria, nitrifying bacteria, and ammonia-oxidizing bacteria. These terms are commonly used interchangeably with nitrobacteria, but they specifically refer to different stages of the nitrification process. Additionally, some species of archaea and fungi also have the ability to perform nitrification, but the term nitrobacteria is typically used to specifically refer to the bacterial species responsible for this process.