tenth: the ordinal that ranks something as number ten, and the one-in-ten portion that follows from it
This is simply the counting word "tenth." Most often it fixes a position in an order: the tenth day, the tenth month, the tenth year of a king, the tenth name on a list, the tenth generation. In the Bible it turns up constantly in dates, because Israel counted its calendar and its official rosters this way.
The same word also names a share rather than a rank. When the feminine form stands on its own it means "a tenth part," one piece out of ten. In the law this is almost always a set amount of flour or grain, "a tenth of an ephah," the standard portion for certain offerings. The wider idea of setting aside one animal or one part in ten also lies behind the tithe.
English "tenth" does both of these jobs too, so the word travels into translation cleanly. The only thing a reader may miss is that the stand-alone "a tenth" in the law is a fixed dry measure, not a loose fraction, and that Hebrew has a separate word for the tithe as an institution.