Civilians are a class of non-combatant units in the game. They are usually unarmed and most of them contribute to resource gathering in one way or another. They are also generally immune to various harmful special abilities, including Sniper (Scouts), Bribe or Informer (both Spies).
The following units are classified as Civilians:
Fishermen, Transport Ships, Merchant Fleets and Modern Merchant Fleets are flagged as Civilians in the game files, however, this actually no effect in the game (Ships are generally immune to special abilities).
Patriots have the Civilian flag as well, making them immune to special abilities.
Note that when Citizens and Scholars are converted to Militia, Minutemen or Partisans, they become combatants (non-civilians) and can be sniped as well. They can also be bribed; however, bribed Militia-type units who were Scholars will revert to Citizens instead. Also, the Korean special Citizen abilities are nation-based and do not transfer when bribed; a formerly-Korean Citizen who was bribed due to being in Militia form will lose the Korean special Citizen abilities when reverted, and a Citizen bribed by Koreans due to being in Militia form will gain the Korean special Citizen abilities when reverted.
Some units are incapable of attacking, like civilians, but have special abilities that make them Support Units. Unlike civilians, support units are considered fair game for snipers, Spies being the only exception, as they have to be killed using the Counterintelligence ability.