one's whole life; one's lifetime
A literary or formal noun meaning a person's entire life. In modern Japanese it is especially familiar inside fixed expressions such as 一期一会, while 一生 and 生涯 are more ordinary for everyday use.
The 一期 in 一期一会 is used in the sense of "once in a lifetime."
He quietly came to the end of his ninety-year lifetime.
生涯 is a formal but still commonly used word for a lifetime, while 一期 read いちご is more archaic and mainly survives in set phrases like 一期一会.
Attested kanji form and reading for the lifetime sense. Be aware that 一期 can also be read differently in other words or contexts.
A Sino-Japanese compound written with 一, "one," and 期, "period" or "term"; for this reading and sense it refers to the single span of a person's life. This is a component explanation rather than a detailed historical origin.