Selah is a Hebrew word found 75 times in the King James Bible. Other than one occurrence in reference to a city (2Kings 14:7), it is used in relation to music. The book of Psalms records Selah the most (71 times) followed by the minor prophet book of Habakkuk (3 times).
Selah is a word meaning, "pause" or the suspension of music that is played (Strong's Concordance #H5542). The Brown-Driver-Briggs Dictionary states it is a technical music term that probably showed accentuation. It is recorded four times in the KJV version of Psalm 89 (verses 4, 37, 45 and 48) and three times in Psalms 3, 32, 46, 66, 68, 77 and 140.
The word almost never appears in the first verse of a Psalm (with the exception of Psalm 67).
To the Chief Musician. On stringed instruments. A Psalm. A Song. May God be gracious to us, and bless us, and cause His face to shine upon us. Selah (Psalm 67:1, HBFV throughout).
I have made a covenant with My chosen; I have sworn to David My servant, 'Your seed will I establish forever, and build up your throne to all generations.' Selah (Psalm 89:3 - 4).
Most scholars agree, according to the 1906 Jewish Encyclopedia, that selah has no grammatical connection with the text. What exact actions the word was meant to signal within Hebrew music is still uncertain.
"It is disappointing after all this to have to confess that of the nature of Hebrew music we have no real knowledge. If any system of notation ever existed, it has been entirely lost" (International Standard Bible Encyclopedia).
Selah may have been a musical notation that marked a crescendo or a musical interlude (Believer's Study Bible). It may have been a demarcation that singing was to cease and the instrumentalists were to play. Tyndale's Bible Dictionary quotes the following from the Talmud that stated what happened when Selah appeared in music sung at Jerusalem's temple.
"Ben Azra clashed the cymbal and the Levites broke forth into singing. When they reached a break in the singing they blew upon the trumpets and the people prostrated themselves;
"at every break there was a blowing of the trumpet and at every blowing of the trumpet a prostration. This was the rite of the daily whole offering in the service of the House of our God."
Did you know . . .
The name Methuselah, which has the word "selah" at the end, means "man of the dart or javelin." He represented the eighth Biblical generation of humans that populated the earth (Genesis 5:21 - 27). Methuselah is also history's oldest human. He is the only person ever to live almost a millennium (he died at age 969), perishing the same year Noah's Flood took place.