define standart deviation
Definition of Standard Deviation. Standard deviation (Ï) is defined as the square root of the variance. It measures the variability about the mean of a data set: the closer to the mean, the lower the standard deviation.efinition of Standard Deviation. Standard deviation (Ï) is defined as the square root of the variance. It measures the variability about the mean of a data set: the closer to the mean, the lower the standard deviation.