ALL CONVERSATIONS WORTH HAVING ABOUT SPACE VOYAGES WERE couched in terms of "delta vee," meaning the increase or decrease in velocity that had to be imparted to a vehicle en route. For, in a common bit of mathematical shorthand, the Greek letter delta (Δ) was used to mean "the amount of change in..." and V was the obvious abbreviation for velocity. The words "delta vee," then, were what you heard when engineers read those symbols aloud.