I was trying to find out when “prince” developed the implicated normal meaning of “heir apparent”, as someone asked. So this site was of course my first port of call.
But there is a curious sentence on here under “prince”.
“As “heir apparent to a throne,” mid-14c. (Prince of Wales ).”
The title “Prince of Wales” given to the future king Edward II was not to identify him as the heir apparent. It was an existing title given to him, previously held by effectively independent princes of Wales. It no more implies that “prince” means “heir apparent” than “Duke of Cornwall” implies “duke” means “heir apparent”.
(And “Duke of Cornwall” is a title automatically possessed by the heir apparent, whilst “Prince of Wales” is not, meaning “prince” in this context is even further distanced from the concept.)
In the fifteenth century, when Owain Glyndwr took the title “Prince of Wales”, I’m sure you don’t think he was claiming to be anything other than an independent leader, not an “heir apparent”! 
Thus, the mere fact that Prince of Wales was a title which was given to an heir apparent is not implication that “prince” meant “heir apparent”.
That’s a common error, which can create quite some confusion in the minds of English people (in particular) towards Wales, so it isn’t the first time I’ve come across it!
So does this statement in the definition have anything else behind it, other than simply the title happening to have been given to someone who happened to be an heir apparent?