The compound 'third-round' is a modern English formation combining the ordinal number 'third' with the noun 'round', denoting a stage. The concept of 'third' originates from the Proto-Indo-European root trei- (three), which evolved into the ordinal forms across Germanic and Romance languages. 'Round' evolved from the Latin rotundus, meaning circular, which broadened in meaning to describe a completed circuit or cycle of activity, such as in competitions or negotiations. The fusion of these elements logically creates a term specifying the third stage in a sequenced process, maintaining a direct link between the literal numerical order and the figurative sense of a cyclical phase.

Structure: third (numerical) + round (stage) 'third' is from Old English þridda, of Germanic origin, related to Dutch derde and German dritte, from an Indo-European root shared by Latin tertius and Greek tritos. It functions as a numerical determiner. 'round' is from Old French rond, ultimately from Latin rotundus 'round', and in this context functions as a noun meaning a stage in a sequence or competition.