Navis
The popular name among the Romans for the reverse of a coin. The origin for this term is naturally to be sought among the Republican issues where the common reverse types to be found on the bronze coins was the representation of a ship's prow. Hence the expression caput aut navis would correspond to uor "Heads or tails."
Source: Frey's Dictionary (American Journal of Numismatics, Vol. 50, 1916)