You misunderstand.
Is not that we don't have a definition. It's that the definition is contextual.
As in,
To whom?
For what purpose?
On what basis?
In what cultural framework?
At which point of history?
etc.

If you were to ask, "how long is a night?" it would be tempting to say 10 hours.
But in Ushuahia, night can last 17 hours.
In some parts of the world there is no sun for three months.
On Mars, a night is much shorter.
And are we counting dusk and dawn? When does day end and night begin?
All of this matters.

My main point is that applying one rigid and narrow definition proves unhelpful, and even harmful.