The compound adjective "ten-year" is a transparent and modern English formation, directly combining two native English elements. The first element, "ten," descends from the Old English numeral, itself tracing back to the Proto-Indo-European root "déḱm̥t," which is the source for the number ten across many Indo-European languages. The second element, "year," originates from Old English "ġēar," denoting a cycle of seasons, derived from the Proto-Indo-European "yóh₁r̥." The compound follows a common English pattern of number-noun combinations used attributively (e.g., five-day, two-hour) to specify duration. The logic is purely arithmetic and descriptive, with the hyphen solidifying the unit to modify a following noun, as in "ten-year plan," where the literal combination of quantity and time period creates a single conceptual unit of duration.

Structure: ten (number) + year (time period) - ten: From Old English 'tēn', 'tīen', from Proto-Germanic 'tehun', from Proto-Indo-European 'déḱm̥t'. Functions as a cardinal number. - year: From Old English 'ġēar', from Proto-Germanic 'jǣrą', from Proto-Indo-European 'yóh₁r̥'. Functions as a noun meaning a period of time.