What is the definition of a swimming lap?

One length is one lap. Webster's II New College Dictionary gives this definition in regard to swimming pools: "2.b. One complete length of a straight course, as in a swimming pool."

I think the confusion comes from a swimmer being "lapped", which is a different term and has a different definition. As a competitive swimmer in summer, high school, and college, we counted 1 length as 1 lap no matter how long the pool, and never talked about "lengths of the pool" at all. My lap counters always counted down from 20 for the 500 and from 64 for the mile (in 25m pools). In the Olympics the 50m is one lap. But, the definition of a lap is not linked to any particular measurement; when you turn around, you are swimming a new lap.

Perhaps we can do away with the confusion by just talking about yardage/meters instead which is how most competitive swimmers talk about how far they swam.

Some people think that 2 lengths of the pool is a lap -- that is from start to the end of the pool and back (seems to make sense, right?), however, according to the rules of the Olympic games, a lap is one length of a pool. This is how competitive high school and college swimmers count laps, as well. In track a lap is one complete distance of the length of track. It's the same in swimming; one length of the pool is one lap.