Priority

A mechanism, established by the Paris Convention for the Protection of Industrial Property of 1883, that enables patent applications to be treated as having been filed on the same day as an earlier patent application that has been filed for the same invention, provided they are filed within twelve months of the earlier patent application.

This process is commonly referred to as "claiming priority to" the earlier patent application and is available in all the contracting states that are party to the Paris Convention (of which there are currently 181 members).  The filing date of the earlier application to which priority is claimed is commonly referred to as the priority date of the subsequent application and determines what constitutes prior art when assessing that application. 

This mechanism enables a commonly used patent filing strategy of filing a first (or primary) patent application in a home jurisdiction and then filing one or more subsequent (or secondary) patent applications within the following year.  This has the advantage of deferring costs until a better understanding of the invention's patentability is gained (via the Search Report for the primary application) and also providing up to an additional year of protection for the invention (since the term of the secondary patents is determined from their respective filing dates not the claimed priority date).