On my local neighborhood website, someone asked for recommendations for someone to clean their house on a ‘biweekly’ schedule.
That is a word that I avoid using because it has never been clear to me whether the user means once every two weeks or twice a week. I looked up Merriam-Webster’s dictionary and it gives both definitions. The Cambridge dictionary says that it means every two weeks in American English and “every two weeks or twice a week” for just English, which is not much help.
Clearly people who use it think it means only one or the other and have never encountered it being used with the alternative meaning. The less common word ‘fortnightly’ clearly means every two weeks and so perhaps that should be used when we want to say that and reserve ‘biweekly’ for twice a week.
But perhaps the best option is to simply say ‘twice a week’ or ‘once every two weeks’ and avoid any confusion.