Then you don't know how to read a definition.
Words can have different independent meanings.
Yes a civilian is defined as
1. A person following the pursuits of civil life, especially one who is not an active member of the military, the police, or a belligerent group.
2. A person who does not belong to a particular group or engage in a particular activity.
3. A specialist in Roman or civil law.
This doesn't mean all civilians are presumed to be experts in Roman or civil law.
It means a civilian is presumed to be in one or more of the three subsets.