What qualifies as retro?

What exactly constitutes as something being labeled as "retro"? Is it simply a matter of age, where older items or styles are automatically deemed retro? Or is it more about the cultural significance and nostalgia associated with a particular era? Does the term "retro" only apply to fashion and design, or can it encompass music, movies, and other forms of media as well? Furthermore, how does the concept of retro evolve over time, as what was once considered modern eventually becomes retro itself?