I came across this formula in a Theory of Interest class. I've never seen this notation before and can't seem to find out how it is meant to be interpreted. What I do know is that it is used for ordinary annuities.
If someone could tell me what its called and how to interpret it that would be much appreciated.
The notation $$ s_{\overline n | i} $$ is shorthand for the expression $$ \frac{(1+i)^n-1}i$$ which is the amount of an annuity of $1$ per payment interval for $n$ intervals. It is usually read as "$s$-angle-$n$ at $i$".
For example of usage, see page 245 of this excerpt.