1A person's entire life span, from birth to death; can also be used adverbially, as in "isshou ~suru" (to do something one's whole life).
祖父は一生を漁師として過ごした。
My grandfather spent his whole life as a fisherman.
彼女とは一生を共にすると誓った。
I vowed to spend my whole life with her.
一生に何度も引っ越しを経験する人もいる。
Some people move house many times over the course of their life.
借金は一生かけて返すつもりだ。
I intend to spend my whole life paying off the debt.
2Used to emphasize something as happening only once, or as being the most significant thing, in a person's life — as in "once-in-a-lifetime" or "the memory of a lifetime."
オーロラを見たことは、私にとって一生の思い出になった。
Seeing the aurora became the memory of a lifetime for me.
これは一生のお願いだから、聞いてほしい。
This is a once-in-a-lifetime favor, so please hear me out.
一生の不覚だったと、彼は肩を落とした。
He hung his head, calling it the biggest blunder of his life.