"And men shall speak of the might of thy terrible acts: and I will declare thy greatness." (Psalm 145:6)
In "Biblical Perspective" Gordon Snider writes:
The psalmist could not find a single adjective to describe the greatness of God. So he added adjective to adjective: “mighty” (v. 4), “wondrous” (v. 5), and here, thy terrible acts. What God does and the way He acts inspires in the righteous reverence and awe. In the unrighteous these same actions inspire fear and dread. The theme of discussion of all generations is the source of His power to do these works. I will declare thy greatness again emphasizes man's public recounting of the mighty acts of God.
Source: Studies in the Psalms: Adult Teacher's Insights, page 77.