In Ancient Greek, the "privative alpha" is a negating prefix, cognate to Latin in- (as in "in-conceivable", not "in-flammable") and English "un-". It survives in English in words like "a-typical" and "an-archy".
But when asking this question, about a name with a privative alpha, I realized there's one important thing I never learned. Is that alpha long or short? And is it consistently one or the other, or does it vary?
Alpha privative was regularly short in Ancient Greek, as shown in Smyth (1920) §885 (a long vowel would have been written with a macron, rendered on the Perseus website as an underscore after the vowel).
Alpha from PIE syllabic n was short as a general rule.
Wiktionary states that ἀ- was treated as long in poetry "when added to a stem that begins with three short syllables", as in ἀθάνατος. That distribution seems a transparent result of poetic license; I don't know however if the long-vowel variant of the prefix has any origin in archaic alternative developments (as poetic variants often do).
