It is a measurement of population per unit area or unit volume.....
BASIC AND BEST :)
The word year is also used for periods loosely associated with, but not identical to, the calendar or astronomical year, such as the seasonal year, the fiscal year, the academic year, etc. Similarly, year can mean the orbital period of any planet; for example, a Martian year and a Venusian year are examples of the time a planet takes to transit one complete orbit. The term can also be used in reference to any long period or cycle, such as the Great Year.[2]
Some of them is the indus river and nile river and the Ohio river hope this helped and please gives points
if you set a precedent then that means to set a example