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:
- "A patentee who makes or sells patented articles, or a person who does so for or under the patentee is required to mark the articles with the word "Patent" and the number of the patent. The penalty for failure to mark is that the patentee may not recover damages from an infringer unless the infringer was duly notified of the infringement and continued to infringe after the notice.
- The marking of an article as patented when it is not in fact patented is against the law and subjects the offender to a penalty. Some persons mark articles sold with the terms "Patent Applied For" or "Patent Pending". These phrases have no legal effect, but only give information that an application for patent has been filed in the Patent and Trademark Office. The protection afforded by a patent does not start until the actual grant of the patent. False use of these phrases or their equivalent is prohibited". 
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.
- UK Intellectual Property Office web site, Display your rights, (under "IPO Home> Types of IP> Patents> Managing your patents> Using and enforcing") Consulted on August 5, 2009.
- IP Australia website, 
- Patents Act 1990 (Australia, Cth) §123
- Patents Act 1990 (Australia, Cth) §178
- United States Patent and Trademark Office web site, - PATENT MARKING AND "PATENT PENDING" (Excerpted from General Information Concerning Patents print brochure)
- 35 U.S.C. 292 False Marking.
- "Watch Out for Patent Marking Trolls"