I have never seen it before and I don't know what variable it represents. Help?
Please always link to where you found this equation. It might well have been an equation that is not common enough to be recognized without the original reference. By chance, it is a very easily recognizable equation, one that has nothing to do with biology.
What your showing is an equation to compute a variance. It is not omicron but sigma ($\sigma$). In statistics, $\sigma$ is the standard deviation and hence $\sigma^2$ is the variance. $\mu$ is the mean and $n$ the number of elements from which these statistics are being computed.