Based on a game by Alice Soft.

One of my classmates says Voltron is not an anime but I said it is anime. Is Voltron an anime or not?

As said before, that depends on your classification and which show. “Anime” in japan simply refers to cartoons, while “Anime” outside Japan refers specifically to Japanese cartoons.
Either way, the first show is an anime and the second, being american, is not. It’s like another famous example, namely the Zweihänder.
Most people call the german swords Zweihänders, while in Germany that word is used to refer to any sword, no matter where it comes from.