What is Patent Pending?

Legal Definition
"Patent pending" (sometimes abbreviated by "pat. pend." or "pat. pending") or "patent applied for" are legal designations or expressions that can be used in relation to a product or process once a patent application for the product or process has been filed, but prior to the patent being issued or the application abandoned. The marking serves to notify the public, business, or 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 a patent pending designation 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.
-- Wikipedia
Legal Definition
Label given to a claimed invention while the Patent and Trademark Office examines the patent application. No protection from infringement exists until a patent is granted, but this label may be used to notify customers and competitors that a patent is being sought. This phrase (or something similar) should only be used if a patent application has actually been filed. A monetary penalty exists for false marking. See 35 U.S.C. ยง 292.
Legal Definition
A simple designation whose basic purpose is to ward off any competitors who might be looking to breach on the copyrights, a 'patent pending' item is one whose patent application is being reviewed.