Trademarks
A trademark registration may remain in force indefinitely, or expire without specific regard to its age. For a trademark registration to remain valid, the owner must continue to use it. In some circumstances, such as disuse, failure to assert trademark rights, or common usage by the public without regard for its intended use, it could become generic, and therefore part of the public domain.
Because trademarks are registered with governments, some countries or trademark registries may recognize a mark, while others may have determined that it is generic and not allowable as a trademark in that registry. For example, the drug "acetylsalicylic acid" (2-acetoxybenzoic acid) is better known as aspirin in the United States—a generic term. In Canada, however, "Aspirin" is still a trademark of the German company Bayer. Bayer lost the trademark in the United States after World War I, when the mark was sold to an American firm. So many copy-cat products entered the marketplace during the war that it was deemed generic just three years later.
Read more about this topic: Public Domain