Priority dates are filing dates that establish the priority of patent applications, especially those filed in more than one country
http://research.hsc.unt.edu/ip_terminology.html

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Note added at 20 mins (2006-08-17 00:14:13 GMT)
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Here's a note that may be of interest
Patent Priority.
This refers to the "priority date" of an issued patent or a pending patent application. Pursuant to National patent laws and Treaties among various countries, someone applying for a patent in one country may claim the benefit of an earlier filing date for the same patent application in another country. The earlier date is referred to as the "priority date". Filing dates always try to claim the earliest "priority date" they can. Also, US patent law permits claiming earlier "priority dates" when filing continuation applications. Basically, a continuation application is a new patent application that contains, at least in part, subject matter which is the same as an earlier patent application. If the subject matter is sufficiently related, the later filed application may claim the benefit of the "priority date" of the earlier filed one.