The 'Napoleonic Code' is a term used to describe the first version of France's Civil Code, which was written and implemented under the rule of Napoleon Bonaparte. It was part of his efforts to consolidate his own grip on France in the period of the Consulate (which he headed); France had, during the course of its Revolution, been in a state of legal turmoil. Successive Revolutionary assemblies had repealed, made, and changed laws constantly, but in the turmoil of Revolutionary politics and the fact that France had been constantly at war from 1792 to 1801, no-one had managed, despite several attempts, to create a single, unified, 'rational' law code for the country.

One of the ways Napoleon made himself more attractive a ruler to the French people and to French politicians than a returned monarchy or a Republic not headed by himself was in restoring France's stability. The Code, along with the Concordat with Rome in 1801 and the effective end of rebellion and brigandage in the western areas of France, was part of this project. The Code was a Civil Code - that is, it was intended to be a complete legal system in its own right, logically and internally consistent. This is in contrast to common-law codes which were widespread at the time, and are still followed in the UK and the US; rather than simply codifying and stating case-law, legal precedent and existing laws, the code sought to rewrite France's legal system into a unified whole. Napoleon tried to use it to impose what he (and many contemporaries) saw as much-needed social order in France: the Revolution's divorce law was rolled back (although divorce was still allowed), for example.

The Code, however, had more significance than that, and its significance lies in the fact that it was exported. By 1812, the French Empire (which the Republic had become in 1804) spanned from the Hanseatic cities, through the Netherlands, 'metropolitan' France, and into the west coast of Italy as far down as Rome, with additional territory in Illyria (modern-day Croatia and its neighbours). As these were legally part of France, they followed the Code, which replaced their existing legal systems. Further, the wider 'Napoleonic' empire, ruled by members of Napoleon's extended family, also adopted the Code: the Kingdoms of Italy and of Naples meant that the Code was theoretically in place in all of the Italian peninsula; the Kingdom of Spain also followed the Code, but only in theory, since much of the country refused to recognise Napoleon's brother Joseph as King when French troops were not occupying the local area; and various German states given to Napoleon's siblings or Marshals followed the Code. Napoleon allowed some deviation according to local norms, but not much. Napoleon also added bits to the code, including commercial law and contract laws, as time went on.

Thus at the end of the Napoleonic Wars, much of Western Europe was possessed of a near-uniform law code designed to fit rationally together - and, as it was mostly still fit for purpose, new rulers often left the core framework untouched. Other rulers adopted its framework but changed the specifics, and it became a model for 'rational' or 'scientific' law codes throughout the 19th Century. This is why it's the basis of not only France's modern civil code, but that of many other European countries - and many of their former colonies, too.

Sources:

The Oxford History of the French Revolution, by William Doyle

The Napoleonic Empire, by Geoffrey Ellis

Napoleon Bonaparte and the Legacy of the French Revolution, by Martin Lyons