Q:

Equation of standard error of a sample mean

In the equation of the standard error of a sample mean, it's given by: standard deviation divided by root-n.
My major concern is: is it the standard deviation of population or the sample on which this is based?
In some books, it is mentioned to be standard deviation of population while some books mentioned it is standard deviation of sample. In Wikipedia, it is the standard Deviation of the population which is impossible to know, hence standard deviation of sample is used. 

A:

In the usual situation, the actual standard error of the mean is $\sigma/\sqrt{n}$, but you almost never know $\sigma$ (the population standard deviation), so you have to estimate it. Consequently, the estimated standard error of the mean is usually taken to be $s/\sqrt{n}$. 
Some sources call that "the standard error of the mean", but strictly it's an estimate of that.

