1A span of one year, or a unit used to express quantities, totals, or durations measured over a year (e.g., in statistics or contracts).
この本は年間30万部も売れているそうだ。
I heard this book sells as many as 300,000 copies a year.
年間契約を結ぶと、月々の料金が少し安くなる。
If you sign an annual contract, the monthly fee becomes a bit cheaper.
彼は年間を通してジョギングを続けている。
He keeps up jogging throughout the year.
このイベントの年間来場者数は年々増えている。
The annual number of visitors to this event keeps increasing every year.
2Attached after an era name to mean 'during the ~ era/period' — a somewhat formal, written-style usage (e.g., 昭和年間 'during the Showa era').
明治年間に建てられたこの洋館は、今も当時の姿をとどめている。
This Western-style building, constructed during the Meiji era, still retains its original appearance today.
昭和年間の写真を見ると、街の様子が今とはずいぶん違う。
Looking at photos from the Showa era, the streets looked quite different from today.