The English word tithe means “tenth.” I’m not sure that’s true in the original Hebrew. However, there has never been any question about the issue - tithe as we read in the Old and New Testaments has always been understood to refer to 10%. This concept was also present in Babylonian and Greek culture. There’s a pretty good article about it in
Wikipedia.
It is not, however, clear that we should now give precisely 10% of our increase. But that’s a topic for a different forum.