noun
many years; long years
Refers to a long period of time that has passed or accumulated. Often used in formal or written contexts.
See also: 昔年
積年の努力が実を結んだ。
Years of effort have borne fruit.
積年の夢が叶った。
A long-held dream came true.
noun which may take the genitive case particle 'no'
long-standing; ancient
Used attributively with の to describe something that has persisted for many years, especially negative feelings like grudges or hatred. The gloss 'ancient' is rare and mostly literary.
積年の恨みが爆発した。
Long-standing resentment erupted.
積年の課題にようやく取り組む。
Finally tackle a long-standing issue.
Compound of 積 (せき, 'accumulate') and 年 (ねん, 'year'), literally 'accumulated years'. The reading is on'yomi. The word is used in formal written Japanese.