Yes, it’s from a book called “Uncle John’s Bathroom Reader” and that book was resting on a closed toilet seat in a nation that only kind of uses the metric system in certain situations… however, you’d be surprised how much you can learn after a single visit to any local fast-food joint!
Precise definition isn’t really practically necessary. It’s only useful if you need to independently recreate a system in isolation. It’s much easier to have a reference material and just copy it. You can even build a reference length if you have something with known measurements.