means A stream or river that flows into and feeds a larger river or lake, rather than reaching the sea on its own.

from From Latin tributarius, 'paying tribute,' which came from tributum, 'a tax paid,' rooted in tribuere, 'to allot or assign' — originally to divide among the tribus, the tribes of early Rome. So the word literally cast smaller rivers as subjects handing their waters up to a greater one, the way conquered peoples handed up their dues.