The difference between two means divided by a standard deviation for the data.
https://en.wikipedia.org/wiki/Effect_size#Cohen.27s_d
μ₁ - μ₂
d = -------
s
_________
s₁² + s₂²
s = / ---------
√ 2
where
μ₁ = mean of sample population 1
μ₂ = mean of sample population 2
s₁² = variance of sample population 1
s₂² = variance of sample population 1
s = pooled standard deviation
This formula uses the common simplified version of the pooled standard deviation.