Is there a term for the point at which returns begin to be diminishing returns?

The law of diminishing returns states that in all productive processes, adding more of one factor of production, while holding all others constant ("ceteris paribus"), will at some point yield lower incremental per-unit returns"

Not necessarily in economic / production theory, except metaphorically. I was thinking about using it in a sentence about further study, or existential dread.

Solution 1:

Point of diminishing returns has been used to describe the point at which the law of diminishing returns kicks in.

Examples from literature:

A very general limitation on the scale of production is found in the economic law of diminishing or non-proportional returns. This law, first applied to land, states that if more and more men are set to work on a piece of land, the time comes sooner or later when the output of the last man added becomes less than the output of the man added before him. This may be called the "point of diminishing returns."

The Brevity Book on Economics, by Harrison McJohnston -- Brevity Publishers, 1919

More recently,

Insulation is a prime example of the "point of diminishing returns" in economics. In essence, the first layer of insulation you put on a home will hold in the most heat and save the most money, while each successive layer will provide less and less of a return for the same amount of investment.

Living Homes: Integrated Design & Construction, by Thomas J. Elpel -- HOPS Press, 2005

Solution 2:

In mathematics, a point where the concavity of a curve changes from concave upward to concave downward is called an inflection point. In more mathematical terms, it is also a point where the second derivative of the curve changes from positive to negative or vice versa, as the example linked in wolfram-alpha shows.

When the marginal returns are increasing, the profit curve is concave up (has a positive second derivative) and when marginal returns are diminishing, the profit curve is concave down (has a negative second derivative). Thus the transition meets the definition of an inflection point.

Solution 3:

If you conceive of returns as occurring along a bell curve rising to a single maximum point and then falling from there, the high point in the curve is the point of diminishing returns—that is, the point at which the returns stop growing and (immediately thereafter) begin to shrink.

This is the supposed insight at the core of the famous "Laffer curve," where the curve in question purports to track tax revenue based on different tax rates and their effects on taxpayers' inclination to earn taxable income. In the Wikipedia article about the Laffer curve, the point at which returns cease to increase and begin to diminish is called the "maximum revenue point."

I suspect, however, that most people who hear or use the phrase "point of diminishing returns" do not immediately visualize it as the apex of a roughly parabolic curve; instead, they probably think of a point at which the value of the returns has become disappointingly small. Such a point is likely to be fairly far down the flank of the curve from the apex, and might be thought of in practical terms as the "point of abandonment." After all, the hardest part of halting any endeavor at the true "point of diminishing returns" is recognizing that point for what it is: unlike, say, a mountain peak, an economic apex isn't easy to identify until you've gone a considerable way down the other side.