The word hemisphere is made up of two parts, hemi and sphere. What does the prefix hemi mean?

Hemisphere originates from the Greek, and consolidates the prefix hemi-, which means "half" with sphere, or "completely round ball." Hemi-is the prefix, signifying "half of a sphere".

Prefixes and suffixes are etymological components that are not autonomous words but rather that change the word root's significance. Prefixes are word parts that show up at the beginning of the word or base. Not every term have prefixes; but rather when introduce, prefixes make the significance more particular.