# What is statistical difference?

In the paper "Calibrating Noise to Sensitivity in Private Data Analysis" by Dwork et al., the term "statistical difference" is used as following (in page 280):

Finally, if a $$1 − \gamma$$ fraction of the $$z$$’s are $$\delta$$-good for a particular pair $$(r, b)$$, then the statistical difference between the distribution $$p'(z)$$ and $$p(z)$$ is at most $$2(\gamma + \delta)$$.

where $$\delta$$-good is defined as:

A value $$z$$ is $$\delta$$-good for a pair $$(r, b)$$ if $$p'(z) − p(z) \leq \delta · p(z)$$.

What is the term "statistical difference" as used in this paper? It seems it is not defined anywhere else in the paper and it is used as if something well-known.