HNO2 is called nitrous acid.
The name nitrous acid is derived from the presence of hydrogen (H), nitrogen (N), and oxygen (O) - three different elements. According to chemical nomenclature rules, the presence of these elements indicates that this is an acid. Specifically, it's named "nitrous acid" because the nitrogen atom is bonded to two oxygen atoms, placing it lower in oxidation state compared to nitric acid (HNO3).
