In terms of the internet sector, it is a narrow sector with low competition and high advertisers. For example, let’s say you are interested in health issues. The word health is a very broad and highly competitive keyword. Therefore, it cannot be a niche sector. If you want to make a site directly on the keyword health, you may have to compete with million dollar sites and your chances of success are low. Instead, you should adjust your keyword to cover a narrower area and lower competition. For example, there may be disease names. Allergy covers a narrower area than the word health, but it is still a broad concept. It can be narrowed in the form of allergies. Even if you just write about one type of allergy, this is a niche area.

There are perhaps hundreds of tools to find your keyword. I specify the free of these tools.
Google Trends
Ubersuggest
Answerthepublic