Until 2015, if you sang "Happy Birthday to you" to your child, you were a copyright infringer, at least…

Until 2015, if you sang "Happy Birthday to you" to your child, you were a copyright infringer, at least that's what Warner/Chappell Music Inc. would have had you believe. The music publishing company had been charging filmmakers and restaurants for the use of the song in movie scenes and birthday parties for decades. In his situation comedy Sports-Night, writer and producer Aaron Sorkin wrote an entire episode about one of the character getting a fictional television network in big legal trouble for singing "Happy Birthday to you" to another on the air. It turns out that song was in the "Public domain", namely that collection of works and expressions that exist beyond the reach of copyright ownership and thus serve as a cultural "common" for all of humanity to use and reuse. No one had ever checked on the matter before about 2010. People just took Warner/Chappell word that the company owned the copyright to the song. It had been earning up to $2 million per year from royalties on the song. In 2010 law professor Robert Brauneis published the results of an extensive investigation of the copyright status of the song and found that, among other technical problems with copyright claim, the song on copyright expired in 1982. As a result of the research by Brauneis, Warner/Chappell faces a lawsuit in an attempt to recover millions of dollars that people have paid to the company for permission to use the song. In 2015 a U.S. Federal court ruled that the song is in public domain, with the court ruling an important part of our culture has been restored to the commons. (250 words) What is the "Public Domain" in the context of copyright?