Other forms: subscripts
A subscript is a character, usually a letter or number, that's printed slightly below and to the side of another character. Subscripts are commonly used in chemical formulas.
A scientist would write the formula for water, H2O, so that the 2 appears lower and smaller than the letters on either side of it. That's what subscript is for, to set certain characters apart from others. It's closely related to superscript, when a letter or number is printed above another character. Subscript comes from the Latin subscribere, "write underneath."