The word 'trackball' is a modern English compound noun, formed in the mid-20th century with the advent of computer input devices. It combines 'track', a word with a long history in English referring to a path or course, and 'ball', an ancient word for a spherical object. The logic of the compound is transparently descriptive: the device is named for its core component—a ball that the user rotates—and its primary function—to track or follow the movement of that rotation to control a cursor's path on a screen. This follows a common pattern in English technical terminology, where two familiar nouns are combined to precisely name a new invention based on its form and function.

Structure: track (path) + ball (sphere) - track: From Middle English trak, from Old French trac, likely of Germanic origin. In this compound, it functions as a noun meaning a path or line along which something moves. - ball: From Middle English bal, from Old English beall or Old Norse bǫllr, meaning a spherical object. In this compound, it functions as a noun denoting the physical object being manipulated.