http://encarta.msn.com/dictionary_1861696741/necessary_evil.html
Basically, calling something a "necessary evil" is one's way of rationalizing the act of doing something unacceptable/evil by saying that it is necessary.
It could also mean something that is absolutely necessary to a country but in a sense it is evil.(truth be told, everything's evil in some sense except for god.)
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Yes
The mathematical term for "mean" is "mean".The popular, or colloquial term for "mean" is "average".
If you mean "What is the adverb for necessary," the answer is necessarily. If not, you may want to rephrase your question.
No, the math term ratio doesn't mean multiply.
There is no statistical term such as "deviation mean".