Is one lap in a pool the length of it and back or is one way only considered a lap?

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.

I think from A to B and back to A is a lap.

As I understand it the definitions are as follows:

“Lap” = A ➜ B ➜ A

“Length” = A ➜ B

When you “do a lap”, no matter what sport, you end up where you started. In a pool that is up and back.

Across and back.

A lap ends when you are back where you started. ” ...one complete turn (as of a rope around a drum.” Merriam Webster

I’ve always wondered this myself. But, it is actually from going back to where you started to finish the lap. I can comfortably do 35 and sometimes a little more of those, in our public pool.
Swimming is great exercise.

A lot of people say lap and mean just the length of the pool. I was taught lap means a full round trip. When I talk to people I always clarify what I mean, I never rely on assuming they define lap the same way I am using it.

It’s like using “downstairs” in a basement house or building. If you are on the main floor and someone says the pictures are on the walls downstairs, do you think the pictures are somewhere on the 1st/main floor or in the basement? It varies around the country. I think we have to be very specific when communicating, and understanding where miscommunications can easily happen.

My daughter is a competitive swimmer and a swim coach. One lap is one length of the pool. There and back is two laps.