What falls under retro?

Could you please elaborate on what exactly falls under the category of 'retro' in the context of fashion, music, or culture? Are we referring to styles, trends, or themes from a specific era that have been revived or re-popularized in modern times? Or does the term encompass a broader sense of nostalgia and appreciation for the past, regardless of the specific time period? I'm curious to understand the nuances and boundaries of this term, as it seems to evoke a wide range of interpretations and associations.