immortalis

Immortalis, immortale is a third declension adjective of two terminations: one form for masculine and feminine, a separate neuter in -e. It is an i-stem, so the ablative singular is immortali, the genitive plural immortalium and the neuter plural immortalia. The prefix is in- assimilated before m, the same negation as in infelix or iniuria.

"Di immortales" is a fixed exclamation, roughly "good God", and it is far commoner in Cicero than any philosophical use of the word. The other place you meet it is the pair with mortalis: gods against men, the deathless against the dying. Church Latin keeps it and adds the soul to the list of things that qualify, which classical usage generally does not.