In the context of scuba diving the meaning of those two words are 100% synonymous. There is literally zero difference.
However, in all languages words carry not only a "technical" definition but also an emotional definition or an "association".
In the case of the word "flipper" a lot of people think of either
- a particular dolphin that was the main character in a television series in the 1970's
- an appendage similar to an arm that can be found on a whale or a dolphin
- something that "flips"... which is to say it raises an association of something that sort of flaps around.
The word "fin", however carries an association as something that
- stabilizes
- controls direction
- increases mobility
Naturally there are regional differences and many different dialects of English; however, because of the "associations" many English speaking divers prefer the word "fin".
Both are "technically" correct words for the things you wear on your feet but the word "fin" seems to resonate better with a lot of people.
R..