In several movies I have seen a pilot or astronaut talking to the ground crew via radio. When the ground crew or tower say’s something like, “…do you copy?”, the pilot responds, “I copy you five by five.” What does “five by five” represent?
It means “loud and clear”, each one being on a scale of one to five.
If for instance, the transmission was loud enough but static-ey, it might be “five by three”.
In Aliens, the pilot announces “we’re in the pipe, five by five,” referring to the fact that the ship they’re in is on course (I’d assume). What would “five by five” mean in that situation?
hmmm…I think it means something like “Perfectly fine”
because I saw it on an episode of Buffy the Vampire Slayer (oh gee, what a reliable source) They asked how Buffy was feeling after she had fought this big-a$$ demon, and she answered “Oh, I’m okay…five by five.” I’ve never been able to lose that phrase out of my memory since…
Actually, Grimmie, it’s not BUFFY’s expression, it’s what FAITH says (the bad slayer.)
Hello? Guys?
Lucretia told you what it meant.
Yes, but in that episode, that line occurred right after Faith and Buffy switched bodies. So, it was Sarah Michelle Gellar who delivered the line, playing Faith pretending to be Buffy.
It is Faith’s phrase, no one else uses it in that show.
Sorta like CB talk when you say wall-to-wall. Just means the reception is clear.
Yeah, but what does it mean in these other situations? Basically, it looks to me something along the lines of “chill, everything’s cool.”
It’s all explained in [url=http://www.randomhouse.com/wotd/?date=20000323]The Maven’s Word of the Day.
I used to have an aunt who everyone referred to as five-by-five (mainly because she coulda bought her clothes in a curtain store. ) But somehow, I don’t think that has anything to do with aliens… 
Meeeeee
Johnny Angel’s link explains it (as did some of the other posts), and you can find a more technical explanation here:
Aviation Glossary.