1. We Have Mentioned Some of the Major Themes of the World Economy
Secular Stagnation - Defining and Measuring It
We Have Mentioned Some of the Major Themes of the World Economy
Most important are (i) low growth, (ii) low inflation, (iii) low interest rates, (iv) a poor "desire" for investment coupled with (v) a strong "desire" to save.
Here is how Lawrence Summers in 2016 has been discussing secular stagnation --

2. "The key to understanding this situation lies in the concept of secular stagnation, first put forward by the economist Alvin Hansen in the 1930s. The economies of the industrial world, in this view, suffer from an imbalance resulting from an increasing propensity to save and a decreasing propensity to invest."
"The result is that excessive saving acts as a drag on demand, reducing growth and inflation, and the imbalance between savings and investment pulls down real interest rates."

3. Short Video (3-4 minutes beginning at 5:32 going to 8:53 ) with Summers on Secular Stagnation

5. Bringing Desired and Actual into Equality is the
Note that Actual Saving Must Always Equal Actual Investment - Always
But
The Desire to Save Need Not Equal the Desire to Invest
Bringing Desired and Actual into Equality
is the
Process of Equilibration

7. Note how that in the video Summers does not say that interest rates have been lowered by central banks, but have fallen due to savers seeking to save more than investors want to borrow and invest. That is, he says low rates are due to a failure of the market, not government monetary authorities.