Subscript

A quantity displayed below the normal line of text (and generally in a smaller point size), as the ``$i$'' in $a_{i}$, is called a subscript. Subscripts are commonly used to indicate indices ($a_{ij}$ is the entry in the $i$th row and $j$th column of a Matrix A), partial differentiation ($y_x$ is an abbreviation for $\partial y/\partial x$), and a host of other operations and notations in mathematics.

See also Superscript