This word is found in the Psalms of David and the book of Habakkuk. Scholars have speculated its meaning since 270 B.C., when the Old Testament was translated into Greek. Since the Psalms were actually songs—set to music—many conclude that selah refers to musicians’ directions. They feel that selah was not a sung word. Rather, it told musicians to play louder or faster, reprise a verse, etc.
But a Jewish music expert, Suzanne Haik-Vantoura, offers a different, more fitting view. Her book, The Music of the Bible Revealed, (written in French) states that selah was not an instruction. In fact, she concludes that it is part of the lyrics. Though she gives no formal definition for the word, she believes that, like amen at the close of prayer, selah stresses the importance or reality of what was said.