As mentioned below, every standing water has (on average) the same amount of inflow as it has outflow, otherwise it wouldn't be there in the first place ("dry up", evaporate, etc.)

In fact, I know an artificial pond (and I guess there are many) that has no obvious inflow, but if you look closely and feel the waters 1-2 m below the surface, you realize it has massive inflow from ground-water (there is a large river just nearby), and there is a swampy area at the opposite end, where the pond has considerable outflow - just not as obvious.

Now we could modify this to say, there must be obvious inflow from a river, stream or similar... But then again, in areas where there are not many big lakes nearby, e.g. Lower Austria, there are several "lakes" (often artificial) which I always jokingly criticize ("You call that a lake?").

If you grew up near e.g. Lake Constance or let's say Lake Michigan, most "lakes" are ponds for you, even if they have inflow.

So I think most people would agree that surface area is the main differentiator, but I would say it is highly subjective where you draw the line.