Meaning and origin of term I've heard in a few movies

Hello,

Was watching Event Horizon last night and kept hearing the crew use the
term
"five by five".  It seemed to indicate that everything is working fine. 
I
thought that I've also heard the term in another SF movie (Aliens,
maybe?).
I was wondering if anyone out there could say what the term means (five
by
five what?) and where it came from.

Thanks,
Nathan