# What is the standard error?

Standard error is our estimate for the unknown parameter $\sigma$ (standard deviation).
$s . e . = \sqrt{{\hat{\sigma}}^{2}}$. It is a measure of the average vertical distance one of our observations is from the calculated regression line. In this way, it estimates the unknown quantity $\sigma$, which would be how far we'd expect any potential observation to be from the actual regression line (the line that we've obtained our least-squares estimate for).