From the 1828 Webster's:
CIVILIAN, n. [from civil.]
1. One who is skilled in the Roman law; a professor or doctor of civil law.
2. In a more extended sense, one who is versed in law and government.
3. A student of the civil law at the university.
From the 1913 Webster':
Ci*vil"ian (?), n. [From Civil]
1. One skilled in the civil law.
Ancient civilians and writers upon government. Swift.
2. A student of the civil law at a university or college. R. Graves.
3. One whose pursuits are those of civil life, not military or clerical.
From the current Webster's:
Main Entry:
ci·vil·ian
Pronunciation: \sə-ˈvil-yən also -ˈvi-yən\
Function: noun
Date: 14th century
1: a specialist in Roman or modern civil law
2 a: one not on active duty in the armed services or not on a police or firefighting force b: outsider
— civilian adjective
The etymology of the word:
civilian
1388, from O.Fr. civilien "of the civil law," created from L. civilis (see civil). Original meaning in Eng. was "judge or authority on civil law," sense of "non-military person" is first attested 1829.
By far the definition, as generally used is:
noun: a nonmilitary citizen
adjective: associated with or performed by civilians as contrasted with the military
So, as we can see, definitions do change over time. The results of the current definition are probably a combination of both the police usage and the citizens usage - It's a chicken and egg question.
Having shown the mess we are in, as regards word usage, I have to say the militarization of the police does in no way help the "Us" vs. "Them" mentality that is prevalent in society today. Nor does legislative enactments that place the police as some kind of "super citizen," help in this regard. sigh.
As Pogo would say: They is us and us is them.