Is hydrogen an atom or molecule?
hydrogen refers to an atom with one proton. But then vocabulary may vary depending on the scientific field.
For some chemists, hydrogen usually refers to a molecule, precisely gaseous molecular dihydrogen H2. For others, it refers to hydrogen atoms bound in molecules.
For astrophysicists, it usually refers to the atomic form H, either ionized or neutral.
But let's mention here something important.
We have similar confusions for elemental oxygen, halogens and mercury.
The term dimer is not convenient neither, because it is commonly used in organic chemistry to characterize two molecules coupled together via hydrogen bonds (for example two carboxylic acids).
Molecular oxygen is confusing too. It obviously refers to O2, but ozone O3 can be dubbed molecular oxygen as well.
We should ideally say dihydrogen, dioxygen, difluorine, etc. as we do in other languages.