Hydrogen is the lightest and most abundant element in the universe, making up about 75% of all matter by mass. It is a colorless, odorless, and highly flammable gas at room temperature.

Discovered by Henry Cavendish in 1766, hydrogen plays a crucial role in stars through nuclear fusion. On Earth, it is found in water (H₂O) and organic compounds. As a clean energy carrier, hydrogen is increasingly important for sustainable fuel technologies and renewable energy storage.