If you look in almost any English - Portuguese bilingual dictionary you'll find that the English word "basil" is translated into Portuguese as "manjericão." One multilingual dictionary translates manjericão into a number of languages such as French (basilic), English (basil), Italian (basilico) and German (basilienkraut). However, when I go to my local supermarket here in Fortaleza, or any fruit and vegetable market anywhere in Brazil, the plant that is labelled manjericão is not what I know as basil, or as basilic or as basilico.
In researching various foods for Flavors of Brazil, I've come to realize that the only wayto make certain that the English word and the Portuguese word for something refers to the same species is to try to find the scientific name for each and see if they are the same. Doing a bit of internet-digging I discovered that I was right - dictionaries notwithstanding, basil (Ocimum basilicum) and manjericão (Ocimum gratissimum) are not identical. They do belong to the same genus, Ocimum, so they're related, which explains the similarity in taste, but they are not the same thing.
My guess is that these two herbs, with similar tastes, differ in choice of climatic conditions for cultivation. The basil that is familiar to English, French and Italian speakers grows best in Mediterranean climates - hot, dry summers and cooler, sometimes rainy winters. The manjericão that Brazilians prefer would seem to be a tropical variety, cultivated here, in Africa and Hawaii.
All of which is interesting from a botanical and linguistic viewpoint, but perhaps less so from a culinary one, as I see no reason why the two plants are not totally interchangeable for all culinary purposes. The only exception might be in something like an Italian salada caprese, where the bright red tomato, white mozzarella and vivid green basil mimic the colors of the Italian flag. In that case, it would be worth walking past the manjericão in the produce section and search out some true alfavaca.