The NCAA doesn't really consider "redshirting" a term that has any official meaning. It's a colloquial term, and occasionally the NCAA refers to it, but with the caveat that it's used to facilitate understanding.

It's fairly simple - 5 years to complete all NCAA eligibility from the first day someone enrolls full time in any 2/4 year college, any place in the world. A student-athlete has 4 years eligibility in each sport. There are also other considerations. A "redshirt" year player can participate in the "non-championship season" (Spring) without using a year of eligibility. A player who exhausts eligibility in four years can also playing in Spring of the fourth year. A player who has exhausted all eligibility in four years can even play in Spring of the fifth year (I've heard of this when there weren't enough warm bodies on a team). I understand a fifth year player without competition eligibility is allowed to practice with a team, and without using up a scholarship from the 12-player limit.

It gets a little more complicated when you're talking medical hardship waivers. That's considered granting an extra year of competition eligibility. I understand it can be difficult to get it approved if a student-athlete has already sat out a year without competition. The NCAA also has the ability to grant a sixth year of eligibility in extraordinary cases.