Subscript

A small letter or number placed slightly lower than the normal text.


Examples:
• the number 1 here: A₁ (pronounced "A sub 1" or just "A 1")
• the letter m here: x_m (pronounced "x sub m" or just "x m")

Often used when we have a list of values.

Example: Average = (x₁ + x₂ + x₃ + ... + x_n)/n
(Which means to add all x values, from 1 to n, then divide by n)


Note: when the letter is up high it is a "superscript".