Definition from Wiktionary, the free dictionary
subscript (plural subscripts)
- (printing) A type of lettering form written lower than the things around it.
- In chemical formulas the number of atoms in a molecule is written as a subscript, so we write H2O for water which has two atoms of hydrogen for each one of oxygen.
- (computing) A numerical index into an array.
Related terms 
a type of lettering form