The modern interpretation of geek is someone who has a strong interest and enthusiasm towards a particular generalized subject, or a specific segment of that subject. There is no restriction on the types of subject that would qualify someone as being a geek on that subject. The interest may be strictly observational, but could also be participatory.
A "nerd" is sometimes used interchangeably with "geek". But more accurately, a nerd is a type of geek (commonly, but not entirely, restricted to interests involving academics or fantasy) who has difficulty communicating coherently with others outside the realm of their own interest.
The subject of a geek's interest can be absolutely anything, such as cars, video games, live theater, movies, music, technology, etc. Or the interest could be focused on a specific segment, such as a type of car, a style of theater, a genre of movies, a style of music, an area of technology, etc.
A music geek's participation, for example, may be limited to just watching musical performances, or actively playing an instrument.
The largest, and most vocal, collection of people who have a strong interest and enthusiasm towards a particular subject would be sports geeks.
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