It isn't defined as a prime number. In my opinion, it should be. Divisible by 1? Yep. Divisible by itself? Yep. No other factors apart from 1 and itself? Yep

It isn't defined as a prime number. In my opinion, it should be. Divisible by 1? Yep. Divisible by itself? Yep. No other factors apart from 1 and itself? Yep. QED