In music, especially, but not exclusively jazz and the blues, a song that has become so popular, or so indicative of a style, that it is covered by many artists over many years.

These songs can be from so-called, popular music, like My Favourite Things, by Rodgers and Hammerstein, or Summertime by Gershwin and Heyward.

They can be composed by jazz greats, like Satin Doll by Duke Ellington.

They can be blues songs, often without any recorded composer, like St. James Infirmary Blues.

It becomes a test of an artist's talent and creativity. Also a test of an artist's ability to program a concert: they are pieces an audience wants to hear, and any program must have a good selection of them to be a success.