OH BOY or BOUY here we go again. Ask a swimmer & one length is a lap.
Ask a round track person & a lap is one traverse of the track not two!!
a dog laps his water bowl & a lap dance goes nowhere!!
A length is always one traverse from one end of the pool to the other.
If we were runners, we'd consider a swimming lap to be two lengths as you start and end a lap at the same point (e.g., the same way you do on a track).
However, we are not runners; we are swimmers and able to have the same word mean two different things. So, sometimes a swimming lap is a swimming length and sometimes a swimming lap is a runner's lap. Embrace both and always be inconsistent when talking with runners or tri-folks so that we don't let them peek too deeply behind the mystical swimmer's veil.