I think the problem, small though it is, stems from the use of a pre-defined word to describe something similar yet different. Almost a colloquialism, where the word is used in everyday speech but not correct in it's formal state.
The term lap comes from Old English 'lappen', refering to a piece of cloth that doubles back on itself, such as a
lapel for example . This doubling back would also insinuate that a lap should be a return to source rather than just one way. The word lap, in English language, is used to describe a circuit. This would also include the out and back in a pool length. (Your own physical 'lap' is effectively where clothing - robes skirts etc, overlaps - hence the name lap)
However, the terminology has been taken and altered with regards to swimming, the Olympic Rules dictating that a lap is a single length of the pool.
Bearing in mind the generally accepted habit of altering the definition of words in the English language to use in a niche setting (i.e. Swimming) then I think the correct term should be 'Lap' for a single length of the pool.
Lap is not the correct term when looking at the English language but it is the correct term for it's niche role in relation to swimming when using the Olympic Rulebook as a standard. If you are using other standards then I guess that meaning applies.
As you can probably tell I looked up some of this, not previously being an expert in the word 'lap'.
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