It is the English term. It collapses the definite article into one word: "the Port" -> "o Porto". Nothing modern uses this name anymore, though. I always see "

It is the English term. It collapses the definite article into one word: "the Port" -> "o Porto". Nothing modern uses this name anymore, though. I always see "Porto", even in American English. But, it's a recent enough fix that you can still find signs of it. For example, you can book a flight to "Porto (OPO)". Just like you can book a flight to "Beijing (PEK)".