Anammox stands for "anaerobic ammonium oxidation". This process is an abbreviation for the nitrogen cycle. Normally, several partial steps are required to turn inorganic nitrogen back into atmospheric nitrogen. Microorganisms doing anammox can oxidize ammonium nitrogen with the intermediate product nitrite (NO2) instead of oxygen (O2) to N2. This process is of great importance for wastewater treatment as it allows nitrogen to be removed from the wastewater with less energy input.
Explanation for children:
Anammox stands for "anaerobic ammonium oxidation". This is an abbreviation: Normally it takes several steps to turn ammonium or nitrite (both circular) back into atmospheric nitrogen (rectangular). With Anammox, the whole thing happens in just one step, so the transformation is much faster. Only very few microorganisms can do Anammox, but they are very important because they also help us with wastewater treatment.