The German translation for arbitrary in its scientific meaning is beliebig or frei wählbar.
The other meanings give it a not intended political twist, but English language unfortunately does not offer a better word to describe exactly that. discretionary gets close, but has unintended secondary implications, too. user-defined would be inexact from a scientific point of view, as we are frequently talking about an application's data being arbitrary in respect to the protocol.
Alright, so now you know about arbitrary wiki pages. ;D