Five by Five

That's military jargon, though I've read it in an old CB operations guide. There are two scales to measure how well you are hearing someone on the radio: Strength and clarity of transmision. Since its a five-point scale, a signal of 'five by five' would be the optimum values. Something worse would be 'two by two' for instance.

So the comment of 'five by five' in this case would indicate that they were feeling in the optimum best of health.

I first heard it in James Camerons' Aliens movie. The female transport pilot says the following during takoff,
"In the pipe, five by five."

It has since been used in the game StarCraft by Blizzard Entertainment. The pilots of the Human shuttle's are female and say this sometimes when you click on them.

Buffy was probably just using it because it became sort of a geek thing to say after the StarCraft inclusion.

all your base are belong to us!

In Alien, Ferro, a dropship pilot, states "We're in the pipe, five by five."

This means in laymen's terms "We're on track and read you loud and clear."

It's usually a military term. There are 5 levels of reception, five being the best. People say stuff like: "Testing, testing, one two three four, five by five." (Or four by four, etc)

Brian and Michael have pretty much hit it on the head. Do note, however that I don't think it's necessarily military. It applies in all forms of radio communication. It is actually an officially documented expression for all licensed FCC radio communications, ham radio, aircraft communications, etc.

It refers to signal strength and clarity, both rated on a five point scale.

http://en.wikipedia.org/wiki/Five_by_five

The other commentators are correct about the 5 by 5 part. I do believe the, 'in the pipe' part refers to being in transit to land. I heard the term originally from Pilots referring to the visualisation produced by the client end of a TLS (Transponder Landing System). The TLS largely automates that landing process.

Yep, as mentioned above it's a radio comm phrase, but turned into slang (like everything else) by the military meaning something like "things are good"

When you answer a radio call you can report back how well you can understand the transmission. You use two ratings, both number from 1 to 5. The first number is the signal strength. The second number is the clarity (amount of static) aka (signal to noise ration).

So for a fair signal you might say 3x3, but if you can hear them loud and clear, then you say 5 by 5, or just 5by, or fiver or who knows what else since I was in.

GroundLimit Out. (not over and out, dunno where that came from ;)

There's a lot of knowledge here. I was a 31L-wiring systems installer, "Cable Dawg", for the US Army Signal Corps. Best job I ever had. Signal is your way of exchanging information. It is measured by Strength and Clarity. Both are rated on a 1 (low) thru 5 (high) scale. 5 x 5 would be as good as it gets. Hoo-ah!

“I understand you completely”
The term 5X5 is rooted in the communications within military forces through WWII to mean "I understand you perfectly." The first number represented the Signal Strength of a radio communication, and the second number represented the signal clarity on a scale from 1-5 -- 1 being the worst; 5 being the best.