terminology - What is this called?

I am looking for the name of an operation similar to factorial.

Factorial would be like:
6! = 6*5*4*3*2*1

but if the operator is "+" and not "*", like:
6+5+4+3+2+1

What would I call this?

Edit:

Sorry, it seems this was a duplicate question. The answer appears to be "nth triangular number". Although that's really not as satisfying and concise as the word "factorial".