Everyone at both CBS and ESPN is slavishly using this term to refer to the third round of the NCAA basketball championships; I haven’t heard “third round”, “round of 16”, or “divison semifinals” used once. I’m not sure why anyone would feel the need to officialize something this minor, but I don’t see any other way it could become this universal.
It’d make more sense if it was always called that, but it wasn’t. In fact, I don’t remember hearing the term at all for the first few years I watched college basketball (around the late 80’s, IIRC), whereas “Final Four” has been around since forever.
NCAA Sweet Sixteen is a registered trademark of the NCAA. Of course since it’s a NCAA trademark, ESPN, CBS and any of the NCAA’s broadcast partners are likely free to use it under their contracts and have legal-speak in place to define the protocols.
Interestingly the trademark is specifically “NCAA Sweet Sixteen” and “NCAA Sweet 16” which implies that “Sweet Sixteen” is likely too widely used in various contexts to be trademarked or is already held as a trademark by someone else. If I had to guess, sweet sixteen by itself is in the public domain. IANAL so I don’t know the specifics.
FTR, this was the 9th item returned when I googled “registered trademark sweet sixteen”. Google is your friend.
Additionally, here you can see the history of the NCAA tourney and it’s format.
The term “Final Four” only dates back to 1978 and was trademarked in 1981.
The field didn’t expand to 64 teams until 1985, which is when the term “sweet sixteen” would most likely have even be relevant. That’s at least part of the reason that Final Four predates it, because the tournament format simply wasn’t condusive to the concept of a final 16 teams for a while there.
The phrase “sweet sixteen” has been around for a long time to refer to other stuff so it’s not unlikely that it was an easy thing for sports-writers to borrow once the current format was adopted. Needless to say neither term is exactly new, but they also haven’t be a fixture with the NCAA tournament forever either.
On a similar topic this link details the trademark disputes that led to the current state of the phrase “March Madness”. Worth noting that it dates back to the 1930s yet wasn’t really associated with the NCAA tournament until early 1980s.
Sweet Sixteen is actually a registered trademark of the Kentucky High School Athletic Association (the NCAA has a licensing deal with them to use it as well.)