In many large cities in the US the starting wage for a fast food worker can be $18+/hr. For someone working in an upscale restaurant can easily get hundreds per day in tips.
$18/hr is also a typical American salary for a new teacher [1], or $25+ with more experience, and I don't think working in an upscale restaurant should pay better.
[1] Many hits on Google.
There are a lot of cultural differences (obviously) but the cost of living is generally less overall. Cafes and restaurants here are often staffed by either younger people still studying (and usually living at home), or older people.
It isn't a route to riches by any stretch, but people get a set minimum wage which should be enough to afford rent/shopping without needing to be supplemented with tips. That said - life is hard and the cost of living is definitely going up here. You wouldn't earn hundreds of Euros in tips even in the best Spanish restaurants though.
There is no definition because it is impossible to define. Everyone will say a number based on their very specific circumstances, problems and expected quality of life (Which will also vary from person to person)
The elephant in the room is that besides trying to define a number for a livable wage there is nothing else but an extreme status quo replacement.