The compound "third-period" is a modern English formation, combining the ordinal number "third" with the time-denoting noun "period." "Third" traces its lineage to the Proto-Indo-European root for "three," evolving through Germanic to signify numerical sequence. "Period" entered English via Latin from the Greek periodos, which originally described a cyclical path or orbit. The logic of the compound is transparently literal, directly applying the sequential meaning of "third" to a defined time interval ("period"), a structure common in sports and academic scheduling to specify a particular segment within a series.

Structure: third (ordinal number) + period (interval) - third: From Old English þridda, from Proto-Germanic þridjô, from Proto-Indo-European tritiyós. Functions as an ordinal numeral. - period: From Middle French periode, from Latin periodus, from Greek periodos ('circuit, cycle, recurrence'). Functions as a noun meaning a distinct portion of time.