Alethea is made up of two parts: a privative "a" and the word Lethe. A privative is a negation. We use exactly the same negation in the words "atypical," "atonal," and "asymptomatic." Lethe, as a proper noun, is the river the dead cross as they enter the underworld. Wading through its waters washes away all memory of their lives in the world, even memory of their self. When they reach the other bank, they are nothing but shadows in the dark. Lethe means concealment, forgetting, oblivian. So alethea means unconcealment, rememberance, and presence. Alethea is a key term in Heidegger's works. He spends great effort contrasting it to the Latin "veritas" which defines truth as the correct correspondence between what's said and what is the fact. His entire lecture series on Logic is devoted to this distinction.
In his Parmenides, Heidegger identifies the goddess who presents Parmenides with the path of true as Alethea. But there is nowhere in the poem itself where that identification is made. She shows P the truth, but nowhere does it say she is truth. If anything she is associated with Dike, justice. I don't think that necessarily weakens H's argument, but it does show one of his tendencies. Heidegger tends to make leaps of imagination and judgement in his interpretations of Greek texts. He justifies it by saying he is trying to understand the Greek in a Greek way.
Coxen, whose text I am using, is more scholarly, though he also notes that alethea is more appropriately translated as "reality" than truth. Coxen's problem, one that Heidegger"s methods were an attempt to correct, is that he has a tendency to too easily gloss the Greek words into modern conceptual language. This is anachronistic. These concepts didn't exist for the Greeks. Their philosophy and their philosophical language presents a struggle, a groping for words to express their ideas. The poetry and the power of these early philosophers is in the struggle. And, as Heidegger continually insists, there were elements, crucial elements, present in that early struggle epithet words and ideas, that we're lost later.