Decennial

Decennial is an English term used to describe a period of ten years or an event that occurs every decade. As an adjective, it modifies nouns to indicate a ten-year span, as in decennial report or decennial census. As a noun, it can denote a ten-year period or a decennial anniversary. The word derives from Latin decennialis, from decem 'ten' and annus 'year'; it entered English in the 18th century and remains primarily in formal or technical contexts.

In practical usage, decennial is commonly associated with censuses or reviews conducted at ten-year intervals. The United States, for example, conducts a decennial census to apportion seats in Congress and guide federal and local planning. Many other countries perform population counts or major administrative reviews at similar ten-year intervals, though the exact timing and scope vary.

Decennial anniversaries or celebrations are often marked as a decade milestone, sometimes called decennial celebrations or decennial events. The term is less common in everyday speech and tends to appear in formal reporting, legal drafting, or statistical documentation.

Related terms denote different intervals, such as biennial (every two years), triennial or tricennial (every three years), and quinquennial (every five years). The decennial concept also appears in statistics, finance, and urban planning, where projects or datasets are organized around ten-year cycles.