Standard Deviation
Etymology
The term was coined by Karl Pearson in 1893. “Deviation” is from Latin “deviare” (to turn off the road), from de- (off) plus via (way), so a deviation is literally a departure from the path.
noun phrase · /STAN-derd dee-vee-AY-shuhn/
A measure of typical distance from the average, expressed in the same units as the data.
What it names
It turns “how spread out is this?” into a single interpretable number, so you can tell the normal from the exceptional.
Example
In many datasets, roughly two-thirds of values fall within one standard deviation of the mean, which is how you gauge whether a result is unusual.
Related idea
A value several standard deviations from the mean is an outlier, and in a normal distribution the standard deviation is what makes “how rare is this?” a precise question.