The word "截然不同" is formed by combining the characters "截" (to cut), "然" (a suffix denoting manner), "不" (not), and "同" (same). The segment "截然" literally means "cutting thus" and acts as an adverbial modifier to emphasize the degree of difference in "不同" (not the same), resulting in the overall meaning of "completely different" through the imagery of a sharp, clear division.