Also, "semi-" might add some quantitative ambiguity. "Bi-" is more specifically a factor of 2, while "tri-" is a factor of 3, etc., though "semi-" can be a loo

Also, "semi-" might add some quantitative ambiguity. "Bi-" is more specifically a factor of 2, while "tri-" is a factor of 3, etc., though "semi-" can be a looser qualifier. If someone said that they did something "bi-annually", that'd sound more like they mean twice-per-year (even if that's an estimated average rate). By contrast, if they said "semi-annually", that'd sound more like they mean usually-several-times-per-year. (Or the inverses rates, due to the other ambiguity.)