The compound adjective 'third-century' is a modern English formation that combines two distinct historical elements to denote a specific chronological period. The first element, 'third', originates from the Proto-Indo-European root \trei- (three), which evolved through Germanic into the Old English ordinal number. The second element, 'century', derives from the Latin centum (hundred), via centuria, which initially referred to a military unit but later, in historical and calendrical contexts, came to signify a span of one hundred years. When combined, the term follows a straightforward logical construction, literally meaning "of or pertaining to the third group of one hundred years" in the Common Era system, thus precisely locating a timeframe between the years 201 and 300 AD.

Structure: third + century - third: From Old English þridda, from Proto-Germanic \þridjô, from Proto-Indo-European \tritiyós. Functions as a numerical ordinal. - century: From Latin centuria (a group of one hundred, a division of the Roman army), from centum (hundred). Functions as a noun denoting a period of one hundred years.