A ballgown is a long dress made of expensive material that a woman wears to a very formal party — historically a ball, a night of music and dancing in a grand room. The dress usually reaches the floor. The cloth is often rich and shiny, sometimes with beads or layers. It is not a short party dress, and it is not the dress you wear to an ordinary dinner. People also say evening gown for a similar long formal dress; ballgown keeps the older picture of the dance. You will see it written as two words, ball gown, just as often as one. The ball here is the party, not the round thing you throw. The noun is the word everyone uses. A verb “to ballgown” exists on paper, but you will almost never hear a person say it.