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The expressions "patent pending" (sometimes abbreviated by "pat. pend." or "pat. pending") or "patent applied for" refer to a warning that inventors are entitled to use in reference to their product or process once a patent application has been filed, but prior to the patent being issued or the application abandoned. The marking serves to notify potential infringers who would copy the invention that they may be liable for damages (including back-dated royalties), seizure, and injunction once a patent is issued.
Fraudulent use of the patent pending warning is prohibited by the law of many countries and inventors should be cautious when marking products or methods that may arguably not be covered by any pending patent application. In some jurisdictions, such as the United Kingdom, a warning notice should ideally mention the number of the pending application.
In Australia, according to IP Australia, the term "patent pending" refers to an invention in respect of which a patent application has been filed at the patent office but for which a patent has not necessarily been granted.  The marking of an article has a legal effect under Section 123 of the Patents Act 1990 with the result that a defendant is taken to be aware of the existence of patent rights. 
In Australia, the preferred marking is "Aust. Pat. App. No. yyyynnnnnn" where "yyyy" is the four-digit year of the application and "nnnnnn" is the six-digit number allocated by the Australian Patent Office.
There are penalties for making a false indication of the existence of patent rights for any invention. 
In the United States, according to the United States Patent and Trademark Office, the expression "Patent Pending" as such does not protect an invention until the actual patent is published and/or issued:
The use of the term "patent pending" or "patent applied for" is permitted so long as a patent application has actually been filed. If these terms are used when no patent application has been filed it is deemed as a deceptive act and a fine of up to $500 may be imposed for every such offense. Under the Forest Group, Inc. v. Bon Tool Co., 590 F.3d 1295 (Fed. Cir. 2009) decision, the current interpretation of "offense" has each mis-marked article constitutes an offense, which permits theoretical damages in the hundreds of millions of dollars for high-volume consumer goods. The Leahy-Smith America Invents Act has changed much of this.