(1) A statistic that reflects the degree of variation in a collection of results. Whereas the range reflects only the difference between the high and low values in the sample data, the standard deviation uses all numbers and therefore reports more information about the data. In small sets of numbers, the standard deviation and the range are similar as descriptions of variability.
(2) A measure of variation in observed values.
(3) Standard deviation, σ, a measurement that defines the spread of data around the average value or mean. The lower the value of standard deviation the better the process is running. Standard deviation of a population is denoted by sigma s, and for a sample it is denoted by s.
A good reference page: Standard Deviation The standard deviation is a statistic that tells you how tightly all the various examples are clustered around the mean in a set of data. When the examples are pretty tightly bunched together and the bell-shaped curve is steep, the standard deviation is small. When the examples are spread apart and the bell curve is relatively flat, that tells you you have a relatively large standard deviation.
Discussion thread: Standard Deviation