This is because its prefix “semi” exactly pertains to half. So, if you have a “semi-annual” event, it means there is a six-month gap between the two. I’m not sure what word came first, semiannual or biannual, but, in the modern day, most people use semiannual to avoid any confusion biannual vs semiannual that arises… Continue reading Biannual vs Semiannual: Demystifying Meaning and Usage