This neuter noun is '[u]sed by the LXX, in the Psalms, for the Hebr. Selah' (LSJ s.v.).
Hesychius, Lexicon delta1440 (a Cyrillian interpolated gloss, according to Latte, deriving from Athanasius, Expositiones in Psalmos 27.69 on Psalm 3.5 LXX) reads: “*dia/yalma [is the term for] when a change in the melody occurs, or a variation in the logic and nature of the speech”. St. Augustine in his commentaries to the Psalms seems to think the same (e.g. Exposition to Psalm IV). But Diodorus Ecclesiasticus, in Commentarii in Psalmos I-L 3.5, considers the dia/yalma only a matter of music and rhythm, not a tool of the text: "now, diapsalma is a musical change and a rhythm variation, not a change in thought, as somebody supposed; the theme of the interlude was sung when, during the song, they changed the melody according to the available instruments. So, as regards changes, these things indicate rhythm variations, not changes in the thought".