Neutral hydrogen refers to hydrogen atoms that have not undergone ionization, meaning they have retained their single electron. These atoms are important in astrophysics, particularly in the study of the interstellar medium and galaxy formation. Neutral hydrogen emits radio waves at a wavelength of 21 centimeters, which allows astronomers to detect and map its distribution in the universe. This emission line is known as the HI line and is used to study the dynamics and structure of galaxies, as well as the properties of the interstellar gas and dark matter distribution. Research in neutral hydrogen includes observations of HI clouds, surveys of galaxies to study their gas content, and investigations of the large-scale distribution of neutral hydrogen in the universe. This field of research provides insights into the formation and evolution of galaxies, as well as the role of neutral hydrogen in the cosmic web of structure formation.