Hastings' Dictionary of the Bible (1898)
SE'LAH. Hab 3:3, Gal 1:9, 2 Kgs 11:13. This is a musical term, and occurs seventy-one times in thirty-nine Psalms, also in Hab 3:3, Gal 1:9, 2 Kgs 11:13 - in all, therefore, seventy-four times in the Bible. The most probable definition of it is that it "directs the falling-in of the sound of the priests' trumpets into the Levites' psalm-singing and playing on stringed instruments. It occurs, therefore, where very warm emotions have been
expressed." "Higgaion," joined with "Selah," Ps 9:16, some render "a louder strain," others, "piano." We know nothing definite about it. See Marginal Reading.