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s as in sd for standard deviation or se for standard error; std as in standard (year or class) in school; possibly others.
The answer depends on the value of the new point. If the new value is near the mean then the new standard deviation (SD) will be smaller, if it is far away, the new SD will be larger.
The area within the normal curve between -1 standard deviation (SD) and +1 SD is approximately 68%. This means that about 68% of the data falls within one standard deviation of the mean in a normal distribution.
http://www.easycalculation.com/statistics/standard-deviation.php This site will calculate SD for you if you have any data
It is not called anything special, just 2 standard deviations or 3 sd.