Flush five is just five of the same exact card rank and suit in case you're wondering
So not only is it a flush since flush = five cards of the same suit
It's also a five of a kind since 3OAK and 4OAK in traditional poker is 3 and 4 of the same rank, and 5OAK which is also its own thing is 5 of the same rank
So therefore it's called a "flush five"