I was reading someone's answer and I came across him using the uppercase x-bar as a notation:
"Technically, the standard error is the standard deviation of an estimator. Most commonly, this refers to sample mean $\bar X$ as an estimator of the population mean 𝜇.
So the 'standard error of the mean' is 𝑆𝐷($\bar X$)=𝜎/𝑛√. If 𝜎 is unknown, it is estimated as the sample standard deviation 𝑆. This means that the '(estimated) standard error' is 𝑆/$\sqrt 2$."