A billabong (/ˈbɪləbɒŋ/, BIL-ə-bong) is an Australian term for 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.

As a result of the arid Australian climate in which these "dead rivers" are found, billabongs fill with water seasonally; they 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". It is derived from bila, meaning "river". It may have been combined with bong or bung, meaning "dead". One source, however, claims that the term is of Scottish Gaelic origin.

Billabongs were significant because they held water longer than parts of rivers; it was important for people to identify and name these areas.

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