What does the term "five by five" mean?

Question

Not as in "5cm x 5cm" but as an answer ('You okay?' 'Five by five.').

Answer 1

Originally it was used to describe the strength of a radio signal in two way communication. If you could hear someone clearly, the signal was strong, and you could turn down the squelch control, then you would say "I'm reading you 5 by 5" Which the maximum units on the oscilloscope that was used to measure the modulation and signal strength. It was Tech speak. Once the laymen started using radios, it was changed to, "I'm reading you loud and clear"... now with cell phones... " Can you hear me now?"