Why is hydrogen called the inflammable air?

Hydrogen is referred to as "inflammable air" because it is a highly flammable gas that can easily ignite and burn in the presence of oxygen. This term historically described its ability to support combustion and produce a vigorous reaction when mixed with air. When ignited, hydrogen burns with a pale blue flame, producing water vapor as the only byproduct. Its low ignition energy and wide flammability range contribute to its classification in this way.