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-!