A billabong (//, BIL-ə-bong) is an oxbow lake, an isolated pond left behind after a river changes course. Billabongs are usually formed when the path of a creek or river changes, leaving the former branch with a dead end. Billabongs, reflecting the arid Australian climate in which these "dead rivers" are found, fill with water seasonally and are dry for a greater part of the year.
The etymology of the word billabong is disputed. The word is most likely derived from the Wiradjuri term bilabaŋ, which means "a watercourse that runs only after rain" and is derived from bila, meaning "river", and possibly bong or bung, meaning "dead". One source, however, claims that the term is of Scottish Gaelic origin.
Billabongs attained significance as they held water longer than parts of rivers and it was therefore important for people to name these areas.