Sure, but that's a bit arbitrary. So I feel free to pick at it :)

Not quite arbitrary, it's mathematically defined via Kolmogorov complexity, the latter just happens to be uncomputable, so we only have approximations using the best available compression techniques. It's good enough for rough estimates.

In principle, maybe. In practice people mean that they can write program P in language A in n lines, whereas they need m lines to write P in language B, therefore A is better than B because m > n.

Which of course means nothing- I can probably write a hello world in 1000 lines in Java, if I try really hard. Just because I can find a shorter program in my favourite language but can't find one in yours, doesn't mean one doesn't exist. And even if you yourself can't find such a program, doesn't mean it doesn't exist.

That's what I mean by "arbitrary": in practice, people come up with whatever interpretation of "expressive" suits them to claim their favourite language is more of that.

> Just because I can find a shorter program in my favourite language but can't find one in yours, doesn't mean one doesn't exist. And even if you yourself can't find such a program, doesn't mean it doesn't exist.

This doesn't matter. Ordering all program strings by Kolmogorov complexity means you can find the shortest program that can produce a certain output string.

I suggest the string "The heat death of the universe should be anytime now".