One's whole life; a lifetime; the entire duration of a person's existence. Can also be used adverbially (e.g., 'throughout one's life').
彼は一生を教育に捧げた。
He devoted his life to education.
一生の友人に出会えたことが何よりの幸せだ。
Meeting a lifelong friend is the greatest happiness.
一生を通して学び続ける姿勢が大切だ。
It's important to maintain an attitude of lifelong learning.
この時計は一生使えるほど丈夫だと言われている。
They say this watch is durable enough to last a lifetime.
Used to emphasize something as the only or greatest in one's life (e.g., 'once in a lifetime', 'the best of one's life').
一生に一度の旅行を家族と楽しんだ。
I enjoyed a once-in-a-lifetime trip with my family.
一生忘れられない思い出ができた。
I made memories I will never forget in my life.
彼女は一生の宝物としてその写真を大事にしている。
She treasures that photo as something to keep for her whole life.