a moment: the briefest span of time, an instant that is over almost as soon as it begins
This word marks the smallest slice of time the Hebrew Bible measures: an instant, a blink, a stretch so short it is gone almost before you notice it. In the singular it is usually adverbial, saying that something happened "in a moment," which very often means "suddenly."
The Bible reaches for it most in two settings. One is how fast disaster can fall: a rebellious people could be consumed "in a moment" (Numbers 16:21), and the proud are "destroyed in a moment" (Psalm 73:19). The other is how short God's anger is set against how long his favor lasts: "his anger is but for a moment, his favor for a lifetime" (Psalm 30:5), and "for a brief moment I abandoned you" (Isaiah 54:7). The same tiny word carries both the terror of sudden ruin and the comfort that the ruin does not last.
In the plural the sense turns from brevity to relentless repetition. God "tests" a person "every moment" (Job 7:18), "waters" his vineyard "moment by moment" (Isaiah 27:3), and terrified survivors "tremble every moment" (Ezekiel 26:16). One instant becomes one instant after another, without let-up.
A close relative, פֶּתַע, stresses the shock of a sudden onset rather than the shortness of the span, and פִּתְאֹם is the plain adverb "suddenly." In the New Testament the nearest counterpart is στιγμή, "a point of time," used of seeing all the kingdoms "in a moment of time."