Longman Dictionary of Contemporary English
Wiktionary
n. Something that cannot be avoided.
Usage examples of "fact of life".
The PN's personnel had been given time to adjust to that fact of life, and while they might intellectually recognize the threat of the Manticoran range advantage, it hadn't come at them completely cold, as it were.
That these parlor games were taken seriously by some people was an inexplicable but inexorable fact of life in the twentieth century.
It was difficult to express clearly since, like any accepted fact of life, one took it for granted and never questioned the fact’.
He accepted that as a fact of life now, had long forgotten the roles of male and female among the Tanu.
Murphy, meaning no disrespect, but it's just a cold fact of life that if I march over to the garrison and start throwing orders around, I'll be lucky if I don't get shot.
And some people say that sneaking out of a hotel is a miracle, particularly if the lobby is swarming with policemen, and other people say it is simply a fact of life, because it happens every day and far too early in the morning.
Let me confront you, though, Captain, with a fact of life that doesn't seem to have penetrated to you at all, though obviously it has to Doctor Wald.
It was a fact of life in the galaxy of the hundred and first century.