The headword, a neuter noun, is best-known to classicists as the pompous comic title of Socrates' academy for paying pupils in Aristophanes, Clouds 94 and passim; cf. under delta 857. It is not attested again until, in later Greek, it is used neutrally of a philosophical school or monastic community.
[1] cf. Hesychius s.v.
[2] This entry seems to be recommending the word for use in Atticist writing. For semnei=on, see sigma 223, theta 228.