终生 (zhōng shēng) means 'throughout one's life' and is used in formal or written contexts, emphasizing a span from birth to death. It is more formal than the common 一生 (yī shēng) and shares a similar register with 终身 (zhōng shēn), which often pairs with specific nouns like 终身大事 (marriage). 一辈子 (yī bèi zi) is the colloquial everyday equivalent.