The prefix semi- means half or partly. When you attach it to the beginning of a word, it tells you that something is halfway or partial.

In geometry, a semicircle is exactly half of a circle, like the shape of a rainbow arching across the sky. A semifinal in a tournament is one of two matches that happen before the final: winners of the semifinals move on to compete for the championship.

Sometimes semi- means “partly” rather than exactly half. A semi-transparent curtain lets some light through but not all of it. Something semi-famous has some level of fame but isn't known by everyone.

You'll find semi- attached to hundreds of English words. A semi-truck is called that because it's only part of the vehicle: the trailer isn't permanently attached to the cab pulling it. When you understand what semi- means, you can often figure out unfamiliar words that use it.