Semi- (half)
Semi-
Half; partly.
Our town has a semi-covered market near the bus stand.
Semicircle
Half of a circle.
The children sat in a semicircle around the storyteller.
Semi-final
The round of a contest just before the final.
Many families watched the semi-final together on TV.
Remember: Semi- = half: a semicircle is half a circle. Some words are joined (semicircle); others take a hyphen in British English (semi-final).