The date that the USPTO receives a complete provisional or non-provisional application. Filing date is very important in determining who gets credit for--and who is entitled to a patent on--an invention. Although America is currently a "First-to-Invent" country--that is, the first to invent something gets any patent that issues, rather than the first inventor to file a patent application--this will change on March 16, 2013. As of that date, the first inventor to file a patent application for an invention will receive any patent that issues, regardless of whether someone else invented the invention before that first filing. This is a massive change in US patent law, and inventors everywhere should be aware of and prepared for it.