Year after year; annually; used adverbially to mean 'each year' or 'from year to year'.
年々、彼の作品はより洗練されてきている。
Year by year, his works have become more refined.
年々、観光客の数が増えている。
The number of tourists is increasing year by year.
年年（ねんねん）と祈りを捧げる習慣がある寺だ。
It's a temple with a custom of offering prayers year after year.