noun
A character or symbol set slightly below the normal line of type, commonly used in mathematical and chemical formulas.
化学式では、原子の数を下付き添字で表します。
In chemical formulas, the number of atoms is indicated with a subscript.
H₂Oの「2」は下付き添字です。
The '2' in H₂O is a subscript.
上付き添字 is a superscript, placed above the normal line, while 下付き添字 is a subscript, placed below.
Compound of 下付き (したつき, 'attached below') + 添字 (そえじ, 'added character').