Divisional patent application

A divisional patent application (sometimes referred to as a divisional application or simply a divisional) is a type of patent application which contains matter from a previously filed application (the so-called parent application). While a divisional application is filed later than the parent application, it may retain its parent's filing date, and will generally claim the same priority. 
Divisional applications are generally used in cases where the parent application may lack unity of invention; that is, the parent application describes more than one invention and the applicant is required to split the parent into one or more divisional applications each claiming only a single invention. The ability to file divisional applications in cases of lack of unity of invention is required by Article 4G of the Paris Convention.
Practice by jurisdiction.
The practice and procedure of filing a divisional patent application vary from jurisdiction to jurisdiction. In most countries, the filing of divisional applications is possible at least as long as the parent patent applications are pending. Since 2013, the European Patent Office rules are again in line with the current practice before the patent offices in the U.S., Germany, and Japan. 
European Patent Convention.
Before the European Patent Office (EPO), divisional applications can be filed under . A European divisional application is a new application which is separate and independent from the parent application unless specific provisions in the European Patent Convention (EPC) require something different.
The practice relating to the filing of divisional applications under the EPC was clarified by the Enlarged Board of Appeal of the EPO in June 2007. The Board held that a divisional application which on filing contained subject-matter extending beyond the content of the earlier application as filed could be amended later to remove the deficiency, even at a time when the earlier application is no longer pending.
United States.
In the United States, a divisional application is seen as a type of continuing patent application, except that if a restriction requirement necessitated the filing of the divisional application, the law provides protection against a rejection of the application and against invalidation of thus-issued patent for double patenting.