A Niche Audience

A niche audience would consume forms of media that are specialised for a particular audience. 
For example, very obscure television channels, like the "SyFy" channel or The History Channel have been constructed to be consumed by only a small variety of audiences. 

In the case of certain TV shows, a series like The Walking Dead would be considered as niche in the UK as it is a survival horror/drama show about a group of people surviving a zombie apocalypse. This most certainly shows that it has been constructed solely for fans of zombie related media a not for a general mainstream audience.

Further evidence that this is a niche show is the fact that it is not shown on any of the most popular channels; the only channel that it is aired on in the United Kingdom is the channel "5*". 

In contrast to the UK, The Walking Dead is an extremely popular mainstream show in America, mainly due to the scheduling and placement of it (what time is it aired and which channel it's aired on), it is shown on the most popular channels at prime time, the time of day when the largest number of people are watching the television. 

If The Walking Dead were to be aired on prime time television on one of the popular channels in the UK, it would be classed as a mainstream show here too.