Answer: Basin
Step-by-step explanation:
If you are dealing with a fairly small area of the land surface, as represented, for example, by a 7-1/2' topographic map, you run into the uncertainty about whether a stream that runs off the edge of your map, along with its drainage basin, runs into one of the other streams that runs off the edge of your map, at a point somewhere outside the area of your map, or into some other stream that doesn’t even show up on your map.