Subscript
Definition
The small number written after an element symbol in a formula. It counts how many atoms of that element are in one unit of the substance.
Technical definition
A subscript is a numeral written slightly below the baseline after an element symbol in a chemical formula, denoting the number of atoms of that element per formula unit. Subscripts are fixed by the compound's composition and must never be altered when balancing equations.
Example
In H2SO4, the subscript 2 means two hydrogen atoms and the subscript 4 means four oxygen atoms per molecule.
Common confusion
Subscripts count atoms inside a formula; coefficients count whole formula units in an equation. Changing a subscript changes the substance's identity: H2O and H2O2 are completely different chemicals.