What is a system?

[Arjan Bos]

Dear list,

For a few years now I've been working around this issue, but it  
finally caught on to me. It might be my lack of understanding of  
musical english, but:
What is a System in for example between-system-padding?

Is it one line of music? Is it one score? is it a set of bars? Is it  
everything that fits on one page?
Could someone please point me to the right part of the documentation  
where system is defined in clear english?

Thanks a lot,
Arjan