Selah: a direction inserted in a psalm marking a moment in its performance, whose exact meaning is no longer known
Selah is one of the genuinely unsolved words of the Hebrew Bible. It appears 74 times, almost entirely inside the Psalms (71 times in 39 different psalms) with 3 more in the song of Habakkuk 3. It is never part of what the psalm is saying; it sits beside the lines like a stage direction, marking a moment when something happened as the psalm was sung or prayed. What that something was can no longer be recovered with confidence, and the most honest scholarship has admitted this for over a century.
A few clues survive. The word clusters in psalms that also carry musical headings (most of the Selah psalms are addressed "to the choirmaster"), which points strongly to a performance setting. The oldest Greek translation rendered it as a musical term for an instrumental break or interlude. And its likeliest Hebrew root means "to lift up," which would fit a cue to lift up instruments or voices. Put together, the strongest case is that Selah was a technical liturgical or musical instruction added by the editors of an early collection of psalms: perhaps an instrumental interlude, a swelling of the music, a pause, or a moment for the worshipers to bow or respond.
Other good readings compete with this one. Some scholars take Selah as a marker of structure in the text, dividing a psalm into sections, rather than a sound cue at all. An old and influential Jewish tradition read it as a word meaning "forever," because Selah often sits near words for eternity (most clearly at Psalm 48:8, the Talmud's proof-text). Most modern scholars regard the "forever" reading as a later guess made by people who no longer knew the original meaning, but it lives on in Jewish prayer to this day.
Two facts make any simple explanation hard. Selah sometimes falls in the middle of a verse, where a pause would break the sense (Psalm 55:19, Psalm 57:3, and twice in Habakkuk 3), and it sometimes stands at the very end of a psalm (Psalms 3, 9, 24, 46, 68), where an "interlude" cannot follow the last line. The ancient Greek translators noticed the second problem and quietly left the word untranslated at those psalm endings. These placements push many scholars away from "pause" and toward either a richer performance cue or a marker of the text's structure.
Because the term means the same kind of thing wherever it occurs, the entry keeps all 74 occurrences together rather than dividing them into senses the evidence cannot support. The real disagreement is not over which verses mean what, but over what the single term signaled in the first place.