noun, adverb
all one's life; throughout one's life; as long as one lives
Used for something that continues for the entirety of a person's life, often in formal or written contexts.
彼は終生その秘密を守り通した。
He kept that secret all his life.
終生忘れられない思い出です。
It is a memory I will never forget as long as I live.
一生 is more common and casual for 'one's whole life'; 終生 is more formal and often used in written language or solemn contexts.
Compound of 終 (end, finish) and 生 (life), literally 'end of life', used to mean 'for the whole of one's life'.