Depends.
In official swimming championships there is a distinction between short and long tracks.
”The FINA World Swimming Championships or “Short Course Worlds” as they are sometimes known, is an international swimming competition. It is swum in a short course pool, and has been held in the years when FINA has not held its main World Championships”
There is the 50 meter butterfly stroke and the 100 meter butterfly stroke.
I would think the 50 (the length of the pool) meter (butterfly, or what have you) is one lap.
When I swim myself, in the pool, I have done one lap when I cleared both up and down the track.