A rooster is a male chicken over a year old and a lot of people call them roos. It's just short for rooster. Personally I don't like the term but it's no big deal.
Technically, a male under one year of age is a cockerel, a male over a year is a cock, or cock bird.
It may not be a big deal here. but I'll bet you that you will never hear a serious breeder use the term, outside of maybe among themselves when telling jokes about what some BYC type person had said. And they do.