noun
long period of time; many years
Refers to a span of many years; often used in contexts like 'for many years' or 'over a long period'.
彼は永年間、この会社で働いてきた。
He has worked at this company for many years.
永年間の努力が実を結んだ。
Years of effort have borne fruit.
長期間 is a general term for 'long period'; 永年間 carries a nuance of permanence or an extremely long duration.
Compound of 永 (long, eternal) and 年間 (period of years). The exact historical derivation is uncertain, but the word is formed from standard Sino-Japanese elements.