Bi- means two. When combined with another word, it means, “every two” or “every other.” For example, bi-weekly means every two weeks; bi-monthly means every two months. Ex: The employees are paid bi-weekly (every other week).
Semi- means half. When combined with another word, it means “twice.” For example, semi-annual means twice a year; semi-monthly means twice a month.
Ex: The employees are paid semi-monthly (twice each month).