subscript
Subscript is a character or numeral that is set slightly below the baseline of a font. Subscript characters are smaller and displaced downward, used to convey chemical formulas, isotopes, mathematical indices, and other specialized notations.
In chemistry, subscripts indicate the number of atoms in a molecule; for example H2O uses subscript 2
In typography, subscripts are typically achieved with a subscript style or markup; in HTML, the element <sub>...<sub>
Unicode provides some subscript characters, such as digits 0–9 and a few letters, but many letters lack
Accessibility considerations: screen readers generally read subscript as normal text, so semantic markup and mathematical markup
Subscripts should not be confused with superscripts, which are placed above the baseline.