The headword is the neuter nominative, vocative, and accusative (and masculine accusative) singular form of the adjective μεγαλεῖος, -α, -ον ; see LSJ s.v., mu 357, mu 358, mu 360, and mu 361. Here, as the context shows, it is neuter accusative, used (in a common Greek idiom) as if it were an abstract noun. The lemma is contained within, and evidently extracted from, a fuller entry already at epsilon 1348 (cf. n. 1-2).
[1] Ennius. Originally from Messapia (Barrington Atlas map 45 grid G4; the region of Salento, southeastern Apulia, in modern Italy), the naturalized Roman poet Quintus Ennius (239-169, OCD(3) s.v., epsilon 1348) wrote an encomium on Publius Cornelius Scipio Africanus (236-183); OCD(3), p. 398; Walbank, p. 191ff; and pi 2054.
[2] Aelian fr.116 Hercher (Domingo-Forasté, p. 87). In her critical apparatus, Adler notes that Asmus had incorrectly located this fragment within Damascius' Life of Isidore (Asmus, p. 448, 479). Gaisford (rashly, by Adler's lights) replaced the headword with Μεγαλόνοια (great intelligence), which appears in the quotation in the accusative singular, and moved the passage so that it followed mu 360; there is no reason to do so, as the headword does appear in the more complete passage at epsilon 1348. Kuster deleted αὖθις ["moreover"], so that this entry conforms to epsilon 1348, and restored the lemma by inserting τὸ μεγαλεῖον after μέτρων (so Adler). Finally, Adler notes that Bernhardy deleted this entry altogether.