(noun) A vessel of chief importance, especially for a key purpose.
"Red five" was Luke Skywalker's vehicle during the final Death Star scene in Star Wars.
"Hey baby, my red five's parked out back, if you want to take a ride."
"Vegetables are the red five for delivering valuable enzyemes and essential nutrients into your body."
"Red Five standing by."
"This is Red Five. I'm going in!"