There is a canonical definition and that's just that there is no more demand-deficient unemployment. Meaning it doesn't even require U3 to hit 0 let alone LFPR to hit 100%. It allows for churn, underemployment as well as unemployables. It's not a metric of achieving utopia, just a situation with almost no slack in the labor market for employers.
https://en.m.wikipedia.org/wiki/Full_employment