Statistics - What is this called in English?

Sp=\sqrt{\frac{\sum_{i=1}^{n}(x_{i}^{2})-\frac{\sum_{i=1}^{n}(x_{i})^2}{n}+\sum_{i=1}^{m}(y_{i}^{2})-\frac{\sum_{i=1}^{m}(y_{i})^2}{m}}{n+m-2}}

What is this thing called in English? Is it variance?