The standard error of the mean is an estimate of the standard deviation of means that would be found if many samples of n items were repeatedly collected from the same population.
An alternate description: Suppose many samples of size n were repeatedly collected from the same population and the means of these many samples were calculated. The means of the samples would themselves form a data set. The standard error of the mean is an estimate of the standard deviation of this theoretical sample of means.
Standard Error of the Mean (SE)
Where
S 

n 
= number of data values (for a sample) 
See Also
95% and 99% Confidence Interval for the Mean
Critical Value of KS Statistic at 90%, 95%, and 99% Significance Level
KolmogorovSmirnov Goodness of Fit Statistic for Normal Distribution