What defines a luxury watch? That is different for every person. There are of course brands that are known as luxury brands, for example Rolex, Omega or Maurice Lacroix. But a watch is often also considered luxurious because of the use of special materials such as precious metals or stones, for example with brands like Cartier or Swarovski.
Other luxury watches are known for special functionalities, such as the pilot watches of Breitling and Alpina or the diving watches of Longines, Oris and Edox. Many classic watch brands with a long history are also seen as luxury brands. Think for example of Frederique Constant, Certina or Tissot.
Another criterion for luxury is the price. We consider € 500 as the lowest price for luxury watches. And finally, exclusivity is also a form of luxury. Some brands are generally not considered to be a luxury brand, which does not mean that some special editions of that brand are indeed considered as luxury watches. For example the MR-G line from G-Shock.