Twice a week or once every two weeks? The English language gets itself into a bit of a twist around this word, but the answer in England is usually that it means twice a week. If we want the other definition then we usually say fortnightly instead, but this is one of the less well known examples of an English vs American division; in the US the phrase biweekly is used roughly half term time for each definition.
However the distinction is made more clear in the case of years. Biannually means twice a year, while biennially means once every two years: notice the difference between the spellings. Until about a year ago I had assumed these two words were just different pronunciations of the same word.
It's all a bit of a mess and in the future using phrases such "twice a week" and fortnightly are probably a good change to make in your life.