As far as I know there are not any standardized terms regarding the traffic on websites. So this here is only my personal opinion:
Several of our customers have website with less then 10 visits per day. I would consider them as very low traffic websites.
For the rest, I think it is easier to measure it in terms of loaded pages per hour/minutes instead of number of visits. You can then break it down to the number of visitors.
Given the fact that a page loads in less than a second I consider all websites with less than one page load per minute as definitely low traffic websites. Usually websites with less than one page load per second are as well considered as low traffic websites.
If you have more than 50 visitors at the same time on your website it starts to become a medium traffic website. If you have more than 5000 visitors at the same time you are getting into the range of high traffic websites.
Then as well it depends on the number of minutes someone stays at the site, etc.