When does bad practice cross the line and become corruption?

Strictly speaking, corruption refers to the abuse of entrusted power for private gain. It includes the obvious: Stealing public funds through accounting fraud or bribery to live a luxury life. But when the abuse isn’t “abuse” so much as it is “flexibility” or when private gain isn’t “private” so much as it is a non-transparent preference that does have some public benefit, is that corruption?