\sum

The TeX (and LaTeX) command for generating the summation sign "∑". When you see this in some ASCIIvision

S(n) = \sum_{i=1}^{n} i = n*(n+1)/2,

it means something like this:

           n
          ---
          \         n (n+1)
   S(n) = /    i  = -------
          ---          2
          i=1

(only you should imagine it with TeX's beautiful typesetting!)

Nowadays, we at E2 have "∑", allowing us to write

S(n) = ∑_i=1ⁿ i = n⋅(n+1)/2.

Not perfect, but decidedly more comfortable than saying "\sum". However, old writeups may well contain the older form; /msging the author (unless it's ariels) may be a good idea.