Snowball is a variation of the swing dance that gets its name from the way that it gathers dancers.
The dance begins with a large number of dancers forming a ring around a small number of dancers (usually around 2-6). When the music starts, only the people in the middle of the ring dance. A little while into the song, the band calls out "snowball!", at which point each of the dancers in the middle grabs a new partner from the ring. This occurs a few more times until everyone has been brought into the dance.
Snowball dances have become a tradition in middle school gyms across much of North America.
References
Snowball (swing dance) Wikipedia(Text) CC BY-SA