I think the word startup has lost all meaning at this point, but I would like to propose this definition: A startup is something you quit your job to work full time on. Until then, it's a side project.
If the OP is fully committed to working on this site full time for the next 3 years, he can call it a startup.
Otherwise, calling a side project a startup seems intellectually dishonest in view of those that are putting 100% effort into a website/startup.
In all fairness, many highly successful businesses started as side projects. In my case, I started my web site about 12 years ago as a side project, and this year I expect to exceed 1 million in revenue.
I'm not sure at what point during the evolution of my side project I went from side project, to startup, to established business.
Maybe a startup should be termed as "a business that is designed to make more money than it spends."
If the OP is fully committed to working on this site full time for the next 3 years, he can call it a startup.
Otherwise, calling a side project a startup seems intellectually dishonest in view of those that are putting 100% effort into a website/startup.