I think the definition of evil changes over time and with different cultures. I don't really know if there can be a universal definition of evil. After all, social progress happens because the societal outlook on moral issues and thus, the definition of "good" and "evil" changes over time. In this sense, "good" and "evil" are mostly labels.

I think a universal definition would look something like:
"A person who is perceived by others as being intentionally harmful to other individuals or groups of such individuals is seen as an evil person."
This is extremely broad though and kind of dodges the whole "depends on the time and place you're in"-dilemma: After all, this definition relies on an action being harmful, which changes meaning over time as well.

Personally, I think everyone has a slightly different definition of "evil", depending on their morals. My own definition would sound like this: "Anyone who aggressively tries to impose their will on another, be it their opinions, world views or agenda ("aggressively" meaning by force, e.g. with violence, intimidation, shaming,...) should be considered evil."

(Prisons, police and the military are a "necessary evil" in my view)