A legal system in which the law is based almost exclusively on legislation.
Such a system is as opposed to a common law system (based on tradition and court decisions) or a religious law system. Civil law regimes tend to be either inspired by or directly descended from Roman legal systems.
According to Wikipedia, “The principle of civil law is to provide all citizens with an accessible and written collection of the laws which apply to them and which judges must follow. It is the most prevalent and oldest surviving legal system in the world.”
Most of Europe and its former colonies have civil law-based legal systems, many of which hearken back to the Napoleonic Code.
This version of civil law is not to be confused with the sort that occurs in the civil law / criminal law distinction, in, among others, U.S. law.