- 1[intransitive] balloon (out/up) to suddenly swell out or get bigger Her skirt ballooned out in the wind. Unemployment ballooned to fourteen percent.
- 2[intransitive] go ballooning to travel in a hot-air balloon as a sport
NAmE//bəˈlun//Verb Forms present simple I / you / we / they balloon
he / she / it balloons
past simple ballooned
-ing form ballooning