Is Thor really immortal?

[Simon]

The apples of Idunn have been used by the Norse Gods to extend their lifespan. Which means that Thor can live to a very ripe old age, why does everyone call him an immortal then?

[Thunderstrike]

Because the Asgardians live a cyclical life. They are born, and die, and reborn to the same life over and over again. It is fated, and destined that way.

Having said that, the Marvel version of Thor is a million times different than that Mythological version, in that Thor's direct heritage from Gaea makes him, for all educated guesses, immortal.

So while the apples of Idunn elongate the lifespans, it doesn't mean that Thor is not an immortal. Plus many (Mephisto, Ego, and Galactus) characters have recognized Thor as being of immortal coil.