What makes a retro watch retro?

Answer 1

Many things and nothing in particular. It highly depends on how you define retro and which era you are thinking about. It's generally a watch with a modern movement (mostly quartz, but sometimes mechanical) and made of modern materials that imitates the look of watches from a certain era. Fossil watches are a good example.

The term "retro" generally refers more to style/look than the movement of a watch. Therefore, a retro watch can look old while having a thoroughly modern movement.

In contrast, a replica watch tries to replicate both the look and the movement of a particular original watch, usually of an expensive and well established brand. The replicated model does not have to be old and the quality of the replica can vary greatly - from a cheap quartz knockoff to a class A replica that can cost over a thousand dollars and is practically indistinguishable from the original .

Retro and replica watches should not be confused with so called "new old stock". Those are genuine (original) watches - usually from a less known but reputable brand - that were manufactured years or decades ago but never sold. In other words, these are more or less old or vintage watches by both style and movement that were just sitting in a warehouse unused and unsold.