In Wikipedia they are called Jötunn. The use of the ö shows that there might not be a traditional name for it in English. As far as I can see from different translations of Volupspa they mostly use the word, Giant, which is not that fitting even though it is used as a word for big or giant in modern Scandinavian. In Swedish you use the word all the time meaning: very. jätte kul = very funny, jätte bra = very good.