noun
Poetic term for a very long time, often used in classical poetry, songs, or formal well-wishing. Can imply 'forever' or 'for generations'.
千代に八千代に、さざれ石の巌となりて苔のむすまで。
For a thousand years, for eight thousand years, until pebbles become boulders and moss grows upon them.
君
May your reign last for a thousand, nay, eight thousand generations.
千年 is the literal 'one thousand years', used in factual contexts, whereas 千代 is poetic and often hyperbolic.
Standard kanji spelling; the most common written form.
Alternate kanji spelling, less common; 世 also means 'generation' or 'world'.
Compound of 千 (chi, 'thousand') and 代 (yo, 'age, generation, reign'). The reading ちよ is a regular rendaku form. The alternate kanji 千世 uses 世 ('world, generation') with the same reading.