noun
Computing term for a character printed slightly below the normal line, such as the 2 in H₂O.
化学式では、原子の数を下付きで表す。
In chemical formulas, the number of atoms is shown as a subscript.
上付き means superscript, where characters are printed above the normal line, while 下付き means subscript.
Compound of 下 (した, 'below') and 付き (つき, 'attached'), literally 'attached below'.