Five By Five 
We have received so many questions from our visitors on what the phrase "Five By Five" really means, that we decided to put the explanation up here permanently. :)
Five by five means that things are good. It is lingo used by the military, radio operators, and pilots about how good/bad a radio signal is, in regards to volume and clarity, on a scale of 1 to 5 (with 5 being the best). So, five by five, means the best possible. So when someone asks Faith how she is? She's telling them she's 'as good as can be'.
The phrase is used in movies like Aliens and Contact.
In 'Aliens', the pilot of the DropShip says it to indicate the first part of their journey was successful and it's smooth sailing from that point on.
Faith mostly uses it in reference to herself. Sometimes physical and always mentally/emotionally.
The interesting thing about Faith's use of the term, however, is that when she says it, it's usually a lie or a gloss over (apart from FHT, when she says it to Oz in reference to his being a werewolf). 
Take for example This Year's Girl: Buffy: "Are you all right?" Faith: "Five by five."
If she means bitter and royally pissed, then yeah, it's the truth. ;) Faith has a tendency to use it when it's not true...to hide her real feelings.
We surmise that Faith picked it up from her main father figure while growing up. He is also the man that Angel represents in Sanctuary.
We always thought it was a father figure, but Sanctuary lends even more weight to that theory. This is specifically the case in the scene where Buffy is accusing Angel of defending Faith. "You did it for her!"
He was more than likely one of her mom's boyfriends, probably her favorite (assuming she had a favorite). Again, we surmise he is military or ex-military. This could also explain Faith's knowledge and love of cool weapons.
By S3 Angel and S7 BtVS, Faith had dropped the saying (sadly, in our opinion). She most likely identifies it as part of herself that she no longer wants to feel or even acknowledge. The 'old' her, if you like. Extrapolating, it's possible that Angel distilled it for her:
" I once told you that you didn't have to go out in that darkness. Remember? That it was your choice. Well, you chose. You thought that you could just touch it. That you'd be okay. Five by five, right, Faith? "
On a side note, it's interesting that the only person to use it other than Faith, was Wes in Salvage. I truly like how Wes has become darker while Faith is reaching to be good. They are becoming each other, in a way.