Original sentence: The reason that S2 is biased stems from the fact that the sample mean is an OLS estimator for μ: it's such a number which makes the sum Σ(Xi–m)2 as small as possible
Edited sentence: The reason that S2 is biased stems from the fact that the sample mean is an Ordinary Least Squares (OLS) estimator for μ: it's such a number which makes the sum Σ(Xi–m)2 as small as possible