In the days of the roman empire, it was a partial citizenship granted to some territories in Italy and other conquered provinces.  It conferred part of the rights a full citizen had, such as the right to enter into a legal contract with a citizen, right to defense in the roman courts and the right to appeal capital convictions.  They could also marry full citizens and be part of the army.  The only right that was denied to them was the right to be part of the roman senate, thus they could not vote, form part of a jury or be elected to any other roman government office.

Source: The First Man in Rome.  Colleen McCullough.  Avon Books.  ISBN: 0-380-71081-1.  Glossary.