a stagnant pool of water in the bed of a stream that flows intermittently
"After the dry season, only small billabongs remained in the riverbed where the creek used to flow."
a branch of a river made by water flowing from the main stream only when the water level is high
"During the heavy rains, the swollen creek overflowed its banks to fill the dry billabong that sits just downstream."
An anabranch, backwater or oxbow lake that is temporarily cut off from the main river, especially one that is only filled with water during the rainy season and can sometimes dry up completely.
"After weeks of drought, the once-vibrant billabong at the edge of the property had turned into nothing but cracked mud."
The word billabong comes from the Wiradjuri language, where it originally described a dead river. It is likely formed by combining words for "river" and "dead."