Here’s a quickie, and it’s actually about prefixes, not words: bi- and semi-. The old way, now dying:
If you wanted to describe something that happens every other year, you’d say it’s bi-annual or biennial.
If you wanted to describe something that happens twice a year, you’d say it’s semi-annual.
Bi-annual (and other bi- words … bi-monthly, bi-weekly, etc.) is replacing semi-annual, which is too bad, since the word is also retaining its original “every other” meaning. That means we’re losing the ability to distinguish quickly and easily between two rather different temporal characteristics. Nevertheless, it is happening — c’est la vie.
But it hasn’t quite finished happening yet. Some people still follow the old ways, so think before you bi-!
The Active Voice 
Imprecise language is the foundation for imprecise thinking.
Maybe the opposite is also true, Harry, and that’s why we’ve got so much imprecise language. :)
biannual means “twice a year” – but does not specify when; “bi-” just signifying two occurrences
semi-annual means “every six months” – “semi-” signifying “half”
biennial means “every two years”
The precision of these three is important in certain contexts, and therefore likely to be preserved.
Hm, Harry here is going to send me to the OED to check this out. The readily accessible online dictionaries I usually use show “biannual” and “biennial” as simple synonyms. Will report back!
I don’t think you’re right about the longevity of “semi-annual,” though, Harry. I almost never hear it used. I think it’s dying.