The Kessler Syndrome refers to a potential nightmare scenario for orbital debris. If the density of objects in Low Earth Orbit  becomes high enough, a single collision could trigger a runaway cascade of additional collisions. The result would be a density of debris that renders Low Earth Orbit unusable for an extended period of time.

The Kessler Syndrome is named after NASA scientist Donald Kessler, who first pointed out the danger back in 1978? In 2009, Kessler wrote a paper on the  significance of the Kessler Syndrome today.

In that paper, Kessler stated the following:

We are entering a new era of debris control…. an era that will be dominated by a slowly increasing number of random catastrophic collisions.   These collisions will continue in the 800 km to 1000 km altitude regions, but will eventually spread to other regions.  The control of future debris requires, at a minimum, that we not leave future payloads and rocket bodies in orbit after their useful life and might require that we plan launches to return some objects already in orbit. 

Emphasis is per the original.

This statement has interesting implications for future space launch. If the key to avoiding the Kessler effect is not leaving rocket components in orbit, the use of expendable launch vehicles  is contraindicated and should be replaced with reusable launch systems as soon as possible.